Why do people feel good after doing something that should be considered as basic humanity?

I’ve noticed that people often feel great about doing things which, in my view, they’re simply supposed to do and that bothers me. For example, if you see a puppy drowning, aren’t you obligated to save it? And if you don’t know how to swim, you should still do everything within your capacity to help. So after saving it, why feel great about it as if it were optional? You did what any decent human being should do.

Take another case, you give your lunch to someone who's hungry, even though you’re hungry yourself. That’s a sacrifice, you’ve given up something, so it's understandable to feel good about your action. But if you only gave them an extra piece of bread when you had plenty, it’s not something to feel proud of. That’s basic decency.

You might say, it encourages people to do more good, but is it right way to move forward?