It's not unusual for dads to dislike the dudes who date their daughters, but More's relationship with his daughter's bae William Roper is a whole different beast. Although he eventually gains respect for Roper, More initially despises him because he lacks the thing that More values most: principles.
We see this embodied in Roper's semi-annual religious flip-flopping. When we first meet him, he's a devout Protestant, despite having been an equally devout Catholic a few months prior. More chews him out from here to Timbuktu for that. And guess what happens? The next time the two men meet, Roper enters proclaiming good news:
ROPER: My views on the Church, I must confess—Since last we met my views have somewhat modified. (1.634)
Translation: "You ran intellectual circles around me, so I can't argue with you, plus I really, really, really want to marry your daughter." Instead of ragging on the dude for changing his views once again, however, More sees this as a teachable moment:
MORE: [...] You see, we speak of being anchored to our principles. But if the weather turns nasty you up with an anchor and let it down where there's less wind, and the fishing's better. (1.717)
But here's one thing Roper does have: passion. After all, Roper is literally the only person who takes More's side in the whole divorce debate—he's also far more eager than More to make a public stink about the issue. Kid's got fire in his belly. Although he might lack a certain temperate nature, Roper is passionate and sincere enough for More to approve of his marriage to Margaret.