understood.That is the crux of the issue isn't it?
For me there are two reasons:
1. Purpose in life. What am I here for? Is this all there is to it? What am I going to do with my life? Responses to those kind of questions are rooted in one of two things. A selfish getting the most out of the time that you have, or a higher purpose. That is not to say that belief in God is necessary for the latter. Plenty of people who do not believe in God are altruistic. But there is a much different mindset when the goal is rooted in what happens after physical death.
2. Loss. I can't imagine the pain of the death of a loved one without that hope of something after a physical death. Complete loss instead of a hope of seeing them again is a horrible thing to endure.
for me, religion provides me [some] hope and guidance in my life now. i think that altruism [whether religious or not] is fully achievable without the expectation of a final reward, and i see the possibility of being altruistic because of love for our fellow humans to be a wonderful and beautiful mindset that does not need the promise of eternal life to come to fruition.
i have lost many loved ones an while it is hard, i also carry on the wonderful time i had with them with me and hope to pass some of that on to those who know me. i don't spend a lot of time worrying about being re-united with lost love ones. again, this is just me.