I’ve never worked in a soup kitchen (although I should, because I think I might enjoy it) but I’ve found that often when I voluntarily engage in a social and purely beneficial activity I enjoy myself enormously. There’s a kind of comraderie going on, it’s like the pleasure of social interaction is combining with the pleasure of Helping in just the right ways.
I don’t expect it would work all the time, or for everyone. And I usually feel differently when I’m forced to do something instead of volunteering. Still, it could be a factor in why some people enjoy that sort of thing.