When you die, you have to come back.
Life is a closed system. You can't get out.
If you believe in heaven or hell, then you come back to life in heaven or hell. Great (or not). But what if you don't believe in heaven or hell?
If you believe in reincarnation, then you return as another living being. But what if you don't believe in reincarnation?
If you believe in only what the rational mind will allow, then your body will decay into the ground and provide nutrients for other life.
Since energy cannot be made or destroyed, the energy we have to live our lives will remain forever and ever. We are basically here to affect that energy positively or negatively. In this way, we're living in our own heaven and hell all the time, every minute, as we make decisions that cause harm or do not cause harm.
My son suggested his own solution to the mystery of death: "When I die, I will knock on everyone's door and they will die too and come with me."
Like my son, I'm not scared of dying, I'm scared of doing it alone. If everyone can't come with me, then at least everybody in the world can come and say goodbye.
Meanwhile, we have to come back!
(unless you eat a lot of preservatives—here's a daunting list— assuming your soul can't go anywhere then your body will never decay)
(oooh. Bummer, the preservative thing is an urban legend, sorry.)