Alright, I'll play ball.
Why help other people? Why be at all altruistic? They're all just a bunch of uncultured, ungrateful bastards who will take advantage of you and see your goodness as weakness.
Right?
Well, we all live in the world. None of us are divorced from the world. We are all in the same boat, so to speak.
Therefore, it follows that if I commit an act of good onto you; I am making the world a better place to live in by improving your life. And as I too live in the world; helping you, helping anyone is the functional equivalent as helping myself. So, by helping as many people as possible I am greatly improving the quality of my own life. Therefore, we can conclude that altruism is inherently both a selfless and a selfish act.
Everything is temporal. Everything. You, me, Earth, the Sun, the Milky Way, the Universe itself? Eventually it will all be gone. Nothing lasts forever. There is no such thing as forever.
So learn to enjoy existing, because it isn't going to be a thing for very long. The Universe has existed for about 13.75 Billion years. Humankind has only existed for a very small portion of that time. You have been around for a significantly shorter period of time, and very soon you will be gone. Everyone you have ever loved, hated, known, or not known will be gone too. No matter what happens to you, good or ill, it's also temporary. Nothing goes on forever, and as long as you remember that you can get through anything.
So that person you still spend time thinking about even though you haven't seen them in years, just because you hate them so much? Let it go. Your time is counting down, and you're spending it hating someone and burning up inside when it doesn't matter and more likely than not never mattered. Does that really sound worthwhile to you?
"They're just as scared of you as you are of them."
"Who?"
"Everyone."