Complex reasons. I would say the first is the knowledge that there is a lot unknown which is mostly subconscious knowledge in Staunch Theists. Then is the security that comes from all your friends and family believing with you. Second one is mostly only a small part. The third one which could be a strong factor is that there is so much evil in the Whole World for which feeling incomprehensible and helpless would be affecting almost everyone except a few. I believe in some kind of Deistic God entity without any Organized religion and my factors are mostly #1 that there is so much we need to seek to know.
Soul can be our colloquial word to call a collection of Information linked with Causal relations. But basically,to mean it as something that is "separate" and "unrelated and outside" "this World" is not true. Information cannot be destroyed. Our minds,Thoughts,memories,etc are actually from Atomic/Molecular and sometimes Subatomic/Quantum level. There is perfect and obvious evidence for this. So as a Transhumanist,I think Soul is as a whole a colloquial term to say information linked with Causal connections.