Why are we on earth?

When we were born maybe or even before that, our destiny, our fate is written, by the one up above in the Heavens...
and as we grow we go through those tasks, phases, moments and minutes and as a speeding car we speed by our years and you open your eyes and you realise that you are no more a little toddler but a young adult... During this journey you find some people whom you like, some whom you don't and a very few with whom you may not be able to live without...
Amidst all this did You ever feel that you have come into someone's life to fulfill something for them in their life? Nope? Then maybe sometime later you perhaps might feel that... Like I did... And you can say that is a very elating feeling that you have, it's like unknowingly itself you have accomplished something really good... If you ever felt the way I'm talking about, you might understand the reason behind this random post =)