what do christians mean when they ask atheists what is their purpose in life? as if no god no purpose?

Question

why is there a purpose for living just because god exists? if he exists is his purpose that you go to heaven if you do what you are forced to do?? so the purpose of living is just going somewhere else when you no longer lived?
and about purposes if live without a purpose is so difficult to understand: what is the purpose of cockroaches, or mosquitoes or flies? if their purpose is just to be food for another animal then just think of human´s purpose for living to be food for worms.

Answer 1

I don't know exactly what christians mean by man's purpose, but I do know what the purpose of the human ape is.
This is our purpose:

http://hrdi.engin.umich.edu/images/Tour%20of%20the%20Solar%20System%20Slides/index.php?file=Moon%20Landing%20(slide%2012)_jpg&num=11

We are the only creature that can go beyond the Earth. If we fail, we are just an ape.

Answer 2

Here is one atheists words on purpose:

“Unless you assume a God, the question of life’s purpose is meaningless.” –Bertrand Russell (20th century philosopher, mathematician and Nobel Laureate)

If we assume God exists, then we should be able to see that God must have put our need for purpose and meaning into our human nature in order to point us to Him. And that is what Christians do believe. A relationship or communion with our Creator gives our lives purpose and meaning and worth just as a child's relationship with the parents is all that is necessary to give the child worth.

We seek God and find God through trying to obey His command to love our neighbors for where love is, there God is also.

Answer 3

well put.