> With regard to the "giving", I wonder what theories there are about why many people feel good when giving things up to benefit others.

Imagine two cultural scenarios: one where everyone is looking out for themselves and helping no one, and another where people are genuinely concerned with each other and eager to help when needed. Which culture would you want to live in?

I think we tend to by default project our motivations onto others (especially in the absence of data). So if we are primarily selfish in our motivations, we think the people around us are selfish. So my guess is that being generous makes us feel like the world is a generous place.

Also, our parents probably taught us that sharing with others is good. So we get a little psychological "stroke" of affirmation about our inner goodness by being generous.