Hard as it may seem to fellows who don't believe in the existence of God, the only reason why we exist is to prepare for eternity. We live in the dimension of time so that we can take care of the available resources and be ready to live forever.
This answer is not easy.
Look at it this way:
Why do students go to elementary schools? To prepare for high school.
Why do they go to high schools? To prepare for life in colleges and universities.
Why do they go to universities? To prepare for life.
And why do they live? To prepare for something greater, eternity.