OK, let's say there is a god (for the sake of argument only - I don't know if there is or is not, and neither do any of you - it's called faith, not fact
)
We have to figure out whether this god is a part of the universe that he created or if this god created the universe but is outside of it. In other words, when the universe collapses, will god cease to exist as well, or will he be sitting on the outside watching it happen? The same question applies to heaven - I've never heard any religion address whether or not heaven is a part of the universe or not. If it's a part of the universe, then even the residents of heaven (angels/spirits/souls/whatever) will cease to exist when the universe ceases to exist and therefore even the religious are just marking time until their own demise - only difference is that they think they won't cease to exist until several billion years from now, whereas those who do not believe in god/the afterlife think they've got 100 years or so if they're lucky.
What it all boils down to is that none of us knows why we're here or what our purpose is or what will happen after we die. I personally don't believe that the biblical version of god exists, and i don't believe there's a heaven waiting for me when I die.
So why don't I just nip off and shoot myself right now - after all what's the point, I'm gonna die anyway so why bother going through the hardship of living? I'd say it's because 1) others don't feel that way and my life may have a positive influence on theirs. 2) why bother shooting myself? I've got all of eternity not to exist, why not make the most of existing while I've got the chance?