The main theme in perks of being a wallflower is that we accept the live we deserve. This line is said twice and anything said twice has to be important. It also connects to Brad and Patrick's relationship, also his sister and her boyfriend and Sam and her boyfriend. With Sam getting cheated on and ponytail derick hitting his sister.
They learn several things like the first night when he meet Patrick was ruined by seeing his sister get hit by Derrick. He learned that anything can ruin a good night. Also he learned that you have to make a move to do anything in life. Several other lessons are feeling the same way others do isn't the same as felling yourself and we can't change where we come from only where we go. The last one connects to Sam and the main character's life.
The title hints that there are perks of being a wallflower but there are none as the movie shows.