Frankie's boyfriend is quite the top dog at their high school. And boy does he know it. The guys got major swagger. In fact, Frankie's "favorite quality of Matthew's was his seeming immunity to embarrassment" (15.8).
Matthew is filled with confidence in who he is and his place at the school. He knows that he's popular, good-looking and intelligent… not to mention wealthy. While we wouldn't go so far as to call him cocky, it's clear he's secure in himself and his position. That means that he's never terribly concerned about what other people think of him, and he always knows the "right" thing to say or do—in his opinion.
Unfortunately, all this swagger makes Matthew more than a little self-centered:
As the weeks passed, Frankie began to see that although Matthew welcomed people into his world with surprising warmth—it didn't occur to him to enter anyone else's. (15.16)
He doesn't think to enter Frankie's world at all because his world is the more important one. He is the man and his world matters more than hers. As his girlfriend, she is accepted only as an extension of him. The folks in Matthew's clique see Frankie on his terms, not on her own. And apparently, she should want to be with him and included in his life because it's obviously more fabulous than hers.
Because he's so confident, Matthew's also got a touch of the white knight syndrome. The first time he actually notices Frankie, it's only because she's taken a spill on her bicycle. And when she says that she's okay finding the gymnasium herself, he asks her to try again:
"Help me, help me. I'm bleeding and I can't find the new gymnasium!" she cried, draping her wrist over her forehead dramatically.
"That's more like it," said Matthew. (7.48-49)
After you get over gagging, consider this: Matthew likes feeling like he's needed. He likes feeling like he's the older protector of someone like Frankie. He sees her as his delicate younger girlfriend who comes in a pretty little package. Because of this, he denies the fact that she's clever, smart, and totally worthy of being a part of his little club on her own terms. But she's a girl, and Matthew only sees girls in a certain way (hint: it's not a good way).
It's not a quality that Frankie loves about him, but she plays along because she knows it's what he wants. And she's only been crushing on him for, like, ever. When she asks him to come home with her for Thanksgiving, she even frames it as an opportunity for him to "save" her from her mother instead of just asking him along as a peer. Say what you want about their relationship, but Frankie clearly knows how to play Matthew the right way.
As a member of the Loyal Order of the Basset Hounds, Matthew's all about his boys. He doesn't see anything wrong with ditching Frankie on a regular basis because Alpha or one of the other guys needs him. Of course that makes Frankie more than a little peeved:
She felt like she never got him alone. Felt like she was always in his world and he was never in hers. (39.25)
Matthew doesn't think to include Frankie in his group because these are the friends he's had forever. He's a part of a pack, and there's nothing that can change that. Here's hoping that eventually he'll learn that he's really just alienating people who, like Frankie, are pretty cool to have around.
But ironically, even though Matthew thinks that he's cooler because of his pack, it actually just ends up proving that he can't do a lot of stuff on his own. Frankie out-thinks the entire pack and comes up with a way smarter Halloween prank, all by herself. She doesn't need a gaggle of loyal followers. She's a lone wolf, and all the more powerful for it.
Because he's a staunch member of the old boys' club, it's tough for Matthew to think outside the box. And that's why he's so disgusted with Frankie when, at the end of the novel, she confesses to having infiltrated the Order. He's not really angry that she betrayed him (although that's part of it). He's angry because "Matthew had called her harmless" (29.18), and she proved to be anything but.
Frankie has defied Matthew's expectations of what a girl should be, and that's why he "hates her" (46.56). All this time, Frankie had hoped "he would understand. […] Admire her cleverness, her ambition, her vision. That he would admit her as his equal, or even as his superior, and love her for what she was capable of" (43.104). But in the end, Matthew can't quite handle Frankie's utter awesomeness. He's just not up to snuff.