If our main motivation was to focus on the pure gains we can receive by giving anything, then we wouldn't even be enjoying giving things to others without receiving anything in return. Sometimes our altruistic acts can cost us ourselves, yet we still do it. The reality of human behavior contradicts the hardheaded absolutist claims by many that we are purely selfish creatures.
It's not either or, we can be motivated mainly by selfish things but that doesn't mean it's exclusively selfish things that motive us. Even altruistic behavior can certainly be selfish in that we are thinking about our selves and how we would like others to treat us in that situation, and because we can do it for the dopamine rewards, or just to feel good about ourselves:
As I see it, sharing enhances relational bonds between psychological beings and enhances the harmony of coexistence thus bring us into a positive emotional state where we feel good about what we are doing, such as being happy. A feedback loop develops between our act of sharing and the generation of that happiness within us. This promotes us to continue to share more.
I think it comes down with free will, fuck with people's free will and they will not be happy with it, here people are happy that they have a choice between pepsi and coke, when they are both equally terrible, which shows that the illusion of choice is enough, but of course point that out and see how quickly their disposition drops. Imposing a will onto others isn't an answer, but I don't think it can be called social norm pressured sharing.
Indeed, mess with free will and people don't like it, whether they were engaging in moral actions or not lol. Many (or everyone) don't want consequences to be brought on them for their immoral actions. They just want to be allowed to continue along with their life lol.
Yes, its multilayered motivations for choices, they are not mutually exclusive, which is the why I counter the usual claim that we are or "need" to act selfishly always, or four whatever we want for our self-interest, regardless of others. Most people don't think that way, so the polarity to an absolute selfish behavior is not usually though of as most of us recognize the reality of our cooperation reciprocal interchange for survival optimization, even if only intuitively it factors into unconscious processing of information feedback to the conscious mind.