We’re out for ourselves, yea. But we’re also out for each other, because we like or need each other. Thinking about things transactionally, that cynicism to avoid getting tricked or hurt… I don’t think that’s conducive to good relationships or a happy life.
This isn't a bad thing. Psychology has shown us that doing favors for someone makes us feel closer to them. Not the asker towards the doer, the other way around. And it's only natural for the doer to balance their own needs against what's being asked, and to expect some sort of reciprocity in the future. This is how trust and relationships are built, at least in primates.