*spoilers*
i dont know if all his problems were to do with how he saw himself, although that had a lot to do with it.
but the film depicts in many scenes that stanley is viewed as nothing- his landord doesnt respect him,the people who fix his car treat him with contempt, his boss at work speaks to him like dirt etc so he is subject to all these things which indicate he is seen to be a loser. its not just a problem with how he sees himself.
when he tries to enter the exclusive club with his friend, he is refused entry and literally thrown to the curb, so its not just people who know him who have a low view of him, he is generally treated like a loser by everyone from all walks of life.
of course we all know he is a decent guy, but the film portrays him as a loser, which as i said in my previous reply is far fetched considering he has job security (a respectable one too), a roof over his head, a car and pet . in any case, the moral of the story seems to be: without a woman/love interest, you are a loser. since in the end that is the only thing he gained. he didnt go after the bad guys because they were going to rob banks, blow up clubs and take over the city, he was only interested in the girl.
i guess its not a film with a deep message, its just a bit of fun, but yeh...