Scott was dumb and a jerk.
I keep hearing this from various people. Having seen the film multiple times, I'd argue that this gross generalization of the character not only doesn't do the character justice, but completely misses the point of the movie. For most of his life he's just impetuous. He's one of those individuals who's empathetic and sensitive, but after incidents of getting hurt or hurting others (through breakups, etc.) learned to distract himself from his problems rather than confront them. Which, unfortunately for Knives, is why he dated her in the first place. When he's in a relationship (prior to Ramona) he feels as though he doesn't have to face his problems. So when Natalie (aka Envy) breaks up with him, not only does he feel rejected for being dumped, but without a relationship, he starts reevaluating his decisions and getting introspective, which he's afraid of doing, because that would mean coming to terms with his prior actions and accepting responsibility. So he hooks up with Knives to distract himself
from himself. Which is frankly the wrong reason to be in a relationship. Because, though he doesn't realize it (since he avoids introspection), he's using her.
His relationship with Ramona, along with his battles against her seven evil exes, serves as a complex and very well done visual metaphor, as Scott not only comes to terms with Ramona's baggage and learns to accept her as an individual despite her faults/baggage, but also comes to terms with his own faults/baggage (his poor relations with Envy, Kim, Knives, and in the books, Lisa, etc.), and mature into an adult.
One of several main plot points of the story is that Scott learns to confront his personal issues and take responsibility for his actions, culminating in the scene where he fights Gideon the second time, hence the line "I think I learned something...", and the whole scene where he apologizes to everyone and accepts responsibility for his actions.
Additionally, it should be noted that Knives is very much a star character in the film, along with Scott and Ramona. Knives' story is one of learning to move on, establish a strong sense of self-esteem and self-respect, and to reevaluate what she values in a healthy relationship.
The third plot is Ramona's, who in some ways is very similar to Scott. She originally agrees to hook up with Scott for the same reasons that Scott hooked up with Knives. She had just come out of a bad relationship, and, rather than deal with her past, she'd rather not talk about it, preferring to just pretend like it didn't happen. She keeps running away from her problems rather than coming to terms with them. Through her relationship with Scott, she's forced to revisit her past relationships, and grows skeptical that she can ever make a relationship work, citing that her past/baggage "keeps catching up with her". When Scott finally matures by the end of the film, he accepts Ramona for who she is, despite her baggage, establishing the foundations for a mature relationship.
The major theme of the film is best summed up in one of Ramona's lines: "No breakup is painless; we all have baggage."
The fact that the characters are so well developed and experience such powerful growth is one of the main factors that make this film worth far more credit than its been given.