The key to understand human motivation

No matter what you do in life or what situation you're in, you often wonder what other people are thinking and why do they do the thing they do, this train of thought often usually represents itself in moments of sadness and frustrations, but asking the question ''why?'' is a natural product of human curiosity that should be fed in order to learn, and while asking why your boss is so rude or why is your wife so mad can have really complicated answers, understanding basic human thinking and motivation is very simple.
When someone does a bad thing, like ethical crimes or legal crimes, we automatically assume that's because he's a bad person with evil intent and an ice-cold heart, if we stop for one second to think why that is we usually come up with a very logical answer: that someone is just doing what he thinks is best for him, it all comes back to his own benefit, selfishness is the root cause of his actions.
But weirdly enough when someone does a good thing we don't ask why, and maybe that's because we're just glad he did, or maybe we are convinced that people do the right thing because they are good people, but the problem with that theory is that it doesn't make any logical sense, for example, why would war heroes sacrifice themselves and go through hell and back for people they never met, and a future they will never see? and why would parents ruin their lives just to make their kids happy? those were the initial question that made the old wheels in my head turn, to answer that I asked why do regular everyday people do good deeds? why are you nice to your teacher or why do you help an old lady cross the street? and the answer to that is either you'll get in trouble if you don't or you do it because it makes you feel good, that feeling of pride and satisfaction, which means eventually you're doing those good deeds for your own benefit too.
And I bet that's the key to understanding humans and why they do both the irrational and rational thing they do, selfishness, self-benefit, it doesn't matter if it's a good person or bad person or some sort of mix of the two, all humans always were and forever will be selfish, that's why parents do the impossible for their kids, it's an instinct that makes them feel responsible, and if they fulfill that responsibility they feel happiness and pride, that's why war heroes sacrifice themselves, they like the idea and the satisfaction of them doing something good and honorable, they might free the country and they might not like the idea of dying but at the end of the day they are doing a good thing because it makes them feel good, why does that make them feel good? because they are good people, because unlike what's common, selfishness is not necessarily a bad thing, it's only bad if used by bad people, when used by good people however, it motivates them to do more good.
What intrigues me about that is that understanding that people do things because of their selfish nature makes trusting people and sharing happiness with others easier, that way you can predict what people will choose in the future, and it makes you love your close ones even more, I mean think about it, if your partner or your friend goes through all the trouble to make you smile, that means that they think that the best way to make themselves happy is to make you happy, even if they don't realize it, that means you two are connected, you have a much stronger bond that goes beyond just making each other happy because you're good people, and that's the beauty of it all.