Before I start, I want to say that I love Sophie, I love Keefe, and this is not meant to be any ship/character bashing. If you don't like stuff like this...please don't read!
1. All my other reasons stem off this: Keefe fits "The Broken Boy" mold TOO well. If you're unfamiliar, "The Broken Boy" is a stereotypical character that shows up in a lot of books. Here's what it is:
This boy has a lot of emotional problems, (We all know what Keefe's are) but hides them under a mask of a certain emotion. (In this case, humor). He eventually opens up the only person he trusts, a girl, (Sophie) who in turn reassures him he's NOT broken. This girl becomes his love interest (Keefe's crush on Sophie) and the broken boy often does reckless things when advised not to. (cough*Running away to the Neverseen*cough)
You see? Keefe fits this SO WELL. He's TOO BROKEN. Sophie and Keefe's relationship, fits this again, TOO WELL, and it's not really a relationship at all, which leads me too...
2. Sokeefe shippers are always like "Sophie and Keefe have absolute trust" but I'm here to tell you, no. Sophie and Keefe's relationship isn't a relationship at all. Ok, listen. All Keefe does is vent his problems to Sophie. (Refer to reason number one.) Sure, I mean he trusts her enough to do that, but he doesn't open up to ANYONE else. Therefore, that's not trust, that is just, again, Keefe dumping Sophie with all his problems, that convieniently he didn't tell anyone else. Then our sweet little moonlark Sophie tries WAY TOO HARD to make Keefe feel better and solve the problem. That's a TOXIC relationship. Ok, think of it this way. If you had a boyfriend/girlfriend, and all they did was tell you all their problems, you would break up in a week. THAT'S NOT TRUST OR A RELATIONSHIP.
3. Their relationship is built on a FAKE Keefe. Ok, even though I said above, Sophie and Keefe don't really have a relationship at all, let's just ignore that for now. Currently, in the books, Sophie and Keefe's "relationship" is based on the funny, outgoing Keefe we all know and love. But, again, refer to reason number one, this is just a mask. If Sophie and Keefe actually get together, eventually, sometime, Keefes facade is going to permanently crack. Therefore, they're building a relationship on a FAKE Keefe, not the broken he is inside.
4. Going back to the "absolut trust" thing, also, Keefe actively DISOBEYS Sophie's orders. Like when he's supposed to stay home while they go on a mission and he doesn't (cough*basically every mission in KOTLC ever*cough). That's not trust. If Keefe doesn't trust Sophie enough to respect her judgement that's GOING TO KEEP HIM SAFE, how can their relationship have "absolute trust"?
Again, this is not Ship/Character-bashing, I love all these characters!