From a teenage perspective, which I presume is where @sashaaa11 is coming from, I don't think it's that boys are complicated it's just that you have to learn how to relate to them. This isn't just a gender issue, it's a growing up issue. There are plenty of adults who lack the emotional intelligence to relate to and understand other people, but when you are young you haven't yet had enough practice, and it's all about practice.
Being able to understand another person or their perspective is often about experience and having interacted with many different people, having experienced different situations and learnt why people are motivated to do certain things. So if you think someone is complicated, try and think about that 'complicated' person's reasons for doing certain things. It's probably that they haven't got those skills of knowing how to relate to another person in a particular situation yet either.
I definitely agree with @litteacher8 in the post above. Sometimes even if a person wants to act in a certain way, they feel they can't and that creates further confusion.