The truth is that for any man to justify existence and make impact on earth, he has to discover purpose.
The question now is: What is PURPOSE?
I will be starting my discussion on the above subject matter right from the Word of God because the Word of God is the beginning and the ending of everything on earth- including human purpose.
Ecc.3v1 says "For everything there is a season, and time to every PURPOSE under the heaven."
Jer.1v5 says "Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet (PURPOSELY) unto the nations."

However, according to Oxford Dictionary, "Purpose is the intention, aim or function of something; the thing that something is supposed to achieve."
Having said all that, the next question now is: how can one discover his/her purpose on earth?

By the special grace of God, I will be writing on ten (9) ways by which one can discover his/her own purpose on earth.

Number 1
1) The first way is to ask God: God is our creator. He is the right person to talk to when we need to know the way forward in life. He says call unto me, and I will answer you, and tell you great and unsearchable things that you do not know- Jer.33v3. The God we serve is a God of purpose. Everything created by him was created for a purpose- animals and leaves inclusive.
Remember, after creating the animals, the next thing He did was to bring them to Adam for naming and identification. Identification based on their individual purposes. So far God is the source of everything in life, He is also the source of our purpose. And as they used to say that the source of a thing is a sustenance of it. We are all from Him, so we should always feel free to call on Him for direction on our purpose in life.

Number 2
2) The next way is to regularly read your bible: one of the ways to discover purpose is by meditating on God's word (The Bible) day and night. This book of the Lord shall not depart out of thy mouth; but thou shall meditate theirin day and night, that thou mayest observe to do according to all that is written therein: for then thou shall make thy way prosperous, and then thou shall have good success- Jos.1v8.
This is very key because without His Word, the world would not have been made. Remember, the world was framed by the word of God. So, that same word is ability endowed to assist us in the discovery of our purpose on earth. Therefore, let us be friendly with the Word of God because nothing was made in the beginning without the Word from God. Remember, everything in the beginning was created by the phrase "Let there be"- for a purpose. (Emphasis is mine)

Number 3
3) The third way is your gift: one easy way to discover purpose is through the use of your inborn gifts and talents. The scripture says "The gift of a man maketh room for him, and bringeth him before kings." Do you like to draw or paint, sing, play musical instrument or dance, participate in sporting activities? What is it you can do that can move you out of the CROWD and place you in the CLOUD and make you sit with the kings and men of substance on earth?
Look inwardly and bring them out because according to one of my pastor friends Pastor Sam Olagbenjo in one of his articles he published four to five years ago, he said "The world will suffer set backs without you"- that is without blessing the humanity with every gift of God in you. Fortunately for us, every of our divine gift are irrevocable- Rom.11v29. So, tap into it and begin to affect your world. Remember, if truly anyone can change the world with his gift, why not you?

Number 4
4) What arouses your interests: This is about what stretches your ears whenever it is either happening or mentioned. Somebody once said that whenever he is talking with his wife and the word EDUCATION is mentioned in the news, his wife will automatically tune off from their discussion and listen to what the newscaster has to say. Many others cannot continue what they are doing immediately it's time for football match.
To others, it's fashion trend. Whatever it is, your responsibility is to discover it and positively affect the world with it. I, for instance always love to talk and write to people on daily basis. In fact, it got to a point in my life when I always feel uncomfortable if I do not bless my followers on the social media with my article that is rife of undiluted word of God. You Too Can Be A Blessing To Many On Daily Basis.

Number 5
5) What people complement you on: Your purpose is related to what you do that touches peoples' heart and meets their need, and most of the time, you do it effortlessly and you enjoy doing it. I, for instance had always knew that I will eventually become a communicator both in talking and in writing.
Why? Because I love to motivate and encourage people on what can be done by them to better their lives even before the assignment of helping people to discover their purpose was given to me by God. If you can examine your life critically, then you will discover that there will always be something that you do and people always compliment you on. That is the pointer to your calling.

Number 6
6) what you can always do when success is guaranteed: This has to do with the knowledge and experiences you have garnered over the years. Life is designed in such a way that you come in contact with or live/serve under somebody or people in certain location, and they make you do things that are not convenient for you till you master the art and science of it. That thing may likely be a pointer to your purpose in life.

Number 7
7) What you notice about things around you: When a statement was wrongly made, a place was not well arranged, the topic of a book does not depict the content, decoration was not well made. What annoys you when not properly done? Your answer to this question is a sign of your purpose. So, painstainkingly look in that direction. God might be talking to you from that angle towards your purpose.

Number 8
8) What makes you to swing to action: What you are passionate about will always make you to do things even when you know you will not be paid for doing it. What is that thing you cannot see and look away from without attending to it. It is your purpose. Even though it may not be fetching you money at the moment, but the more you persist in doing it, the more it begins to put money into your pocket, and food on your table.
Also to balance it, don't be too focus on the INCOME you are going to get from it, but think of the effect of its OUTCOME on the humanity. Remember, Joseph had to interpret a dream for FREE before he eventually interpreted another dream for a FEE of uncommon position in a Egypt- which was the most powerful economy in the world as at that time.

Number 9
9) And last but not the least, what and who you admire: There is a connection between what and who you admire and your purpose in life. If you admire anything made with gold, you are likely to flourish doing gold related business and if you admire somebody because of what he does or the position/level he has attained, you have what it takes to also get there in a way that is JUST.

However, having mentioned and explained ways through which purpose can be discovered in life, it is my pleasure to also mention a few factors that may likely be a threat to our purpose on earth.

1) Self: The only one that can limit you in life is yourself. No mountain is too high for a determined mind to climb. But a lazy man will  never attempt to work on himself with the view to bring out every godly potential in him. Believe in God, and also in yourself.

2) Friends: while growing up and climbing the ladder of growth in life, my parent use to tell me to always watch out for the type of friends that I have. He said there are two types of friends: those that will make you, and those that will mar you. Many friend will never believe in your purpose hence the reason they will do everything to derail you from manifesting it on earth. Which is the reason the bible says "Evil communication corrupt good manner."
Solution to this: Prayerfully walk away from them.

3) Parents: Research has shown that in some of the advance countries, some of their children are always allowed to decide on what they want to become in life. But it is most unfortunate that we are still having some parents using their authority over a child and coerce him to go and study the course that is not related to his purpose in tertiary institutions thereby waste a whole lot of the child years of fulfilment.

Solution: Tell God to touch the heart of your parents because the bible says the heart of the king is in the hand of the Lord.

And finally,
4) Mentor/Pastor: Many Mentors & Pastors who are charlatan and impostor have derailed many people from their divine purpose on earth. The fact that someone is a man of fame does not mean he should be chosen by you as a mentor. And the fact that someone is a seer does not mean he can actually see what your purpose is all about. We have to be very careful with whom we chose as our Mentor & Pastor in life. We have to first of all pray to God before going to them.
Remember, age is never a barrier in the discovery and fulfilment of purpose on earth. While some people became the M.D. Of their company at age 25, others never become theirs until they are 50 years.