Human Nature - what is it?

I think human nature is just how we as people are and how life is, because we made it that way. I think that people at their core are selfish, because no one truly thinks about others. For example, when people pray for things to god, such as, “help me get through this.” Does anyone else think that there are bigger problems in the world such as cancer and world hunger? No, they are focused on helping themselves. No one ever just sits down, and thinks about other things. Yes, I understand that people do pray for other things, like other people but I wasn't talking about that situation.
If that example doesn't make sense I have another one. People always think that they are right when it comes to problems and life in general. For example, if you were to invite some people over to a party that didn't get invited to another one, and the person that didn't invite you or the people to the other one just showed up, you would think that you were right in that situation. You would think, "Hey, I was being nice and the other person is just a jerk for doing what they did." while the other person is like, "Ugh they are so rude to not invite me how lame." or something like that, If you were anyone of those people, you would think you were right, right?
People are selfish no matter what, and nothing can change that. Even if you are reading this right now, you might think, "whatever this is stupid." or something but this is all true if you just sit down and think about it. When was the last time you didn't worry about that History test and just thought about starving people living on the streets?