Have you ever....?

Allright, just hear me out. Have you ever been in a situation like this: Imagine this scenario; you see this person and the person notices you but completely ignores you. You might say that its normal but what if it happens again and again? Dosen't it bother you? It sure does bother me.... Maybe its just me but I just don't understand it. Why do people act a certain way when they meet someone they don"t know or when someone smiles at them. Is it human nature to do so? Just something to think about...