Why does Oscar's life fall apart after he sees Maritza with Nelson?

It's fair to say that Oscar doesn't have much luck with the opposite sex. From an early age, he's awkward around girls, which makes him the subject of ridicule among his peers. He tries to overcompensate for this by dating two girls at the same time: Olga and Maritza. But Maritza's not exactly thrilled at the arrangement and gives Oscar an ultimatum: either he stops seeing Olga, or she's finished with him. Not being the strongest of characters, Oscar gives in to Maritza's demand and dumps Olga.
Unfortunately, he is himself dumped in due course by Maritza, who seems to have the hots for a boy called Nelson Pardo. Oscar's absolutely heartbroken at being given the elbow; what little confidence he had has been completely shattered and his life quickly goes downhill. As well as gaining weight and developing zits, he retreats further and further into a geeky fantasy world of sci-fi and comic books. This is Oscar's comfort zone, the only place where he feels safe in a harsh, incomprehensible world.