When we study humans, we generally find patterns of behaviour. It is these patterns that allows us to formulate ideas about the human mind and human behaviour. But none of these ideas are ever proven or disproven. The best we can do when it comes to humans is find evidence "for" or "against", but nothing is ever cut and dry. And why do we study it? People study it for the love of studying it. It's like arguing philosophy. You will never get any definitive answers, but they do it for the love of postulating theories.
Psychology is considered one of the social sciences, and is usually distinct from "pure" science. Unlike pure science, who's results are reproducible exactly with every single time the test is conducted, social science will rarely get the same result even once.