Reason for existence: FUN, FUN, and more fun. Any way you can get your kicks, get them, as long as your kicks don't deliberately hurt others. I think the reasons for existence have changed throughout history. There have been times where existence was just hunte, gather, farm, procreate, and sleep. During those times, people still found outlets to have fun, but the lack of convenient living available to them made fun and the time available for fun lacking.
Today, there is no excuse for not having fun, whether it be posting on a message board, playing your favorite video game, playing with the chemistry of your mind with some substance of ill repute, having sex with life partner or multiple partners, having kids and raising them and hoping they grow up free and healthy and sane, ect. Yes, three meals and a roof are important so earning a living is important. Still, the ultimate reason for existence should be fun, fun, and more fun, with work being used as a tool to facilitate fun.
The people that like to attribute larger then life reasons for existence are those that are scared of the unknown and death. Just enjoy life for as long as you've got it.