One of the first things in psychology they tell you is that...There is no one "big" theory that can explain everyone, their thoughts, their feelings and their actions.
There are many theories that you do learn about. Each theory has their good points and bad points. From all the psyc classes I have taken, the theory that I believe in most is basically the middle of all the "extreme" theories.
This is the theory of nature and nurture. It seems most recent pyschologists feel the same way and that although genetics does play a role in people's cognitive development, nurture is either equally important or maybe even more.
In addition I just want to point out that in psychology you don't just learn how people behave...There are many branches. For example...Cognitive development and then that can break into infants congitive development and childrens cognitive development...You can also learn about what parts of the brain does what...Or how memory works. In my university it is also mandatory to take a couple of psychology statistics courses as well...So you do learn some math.