Xander often times acts as though being such a decent person entitles him to the type of attention from women that he desires, certainly from someone, if not from Buffy herself. It certainly should entitle him to dictate who is and isn’t good enough for Buffy to devote her attention to since it isn’t going to Xander. After all, who is this Angel guy? It isn’t Xander! How can he possibly be good enough for her? He couldn’t even save Buffy when she died! He does this repeatedly with both Buffy and Willow, criticizing both Angel and Oz, and often trying to go tete a tete with Angel over Buffy’s maidenhood… or something.
Xander continues to display his staggering Nice Guy™ personae even after he enters into his (at first secret kissy-face) relationship with Cordelia. When he isn’t attached to Cordy at the lips, he is talking incessantly about Buffy to her, or later, Willow, ignorant of the perfectly wonderful (admittedly flawed) person in front of him. Xander’s treatment of Cordelia is heartbreakingly cruel (and probably indicative of what is to come, because Joss, I have a major beef with you and your treatment of Cordy). After Cordelia stands up to her friends in order to keep her relationship with Xander, he repays her in kind by betraying her with a person he didn’t really want to be with until he couldn’t have her.