Not because he didn't like Molly. And not because he didn't want Sam to be happy. But because the girls Sam took up with... well, aside from the one girl, when they were looking into that auction house the one time... they were all sorta buckets of crazy. Or they died.
After Ruby, Dean didn't ever want to see Sam with another girl with any kind of supernatural ability. Nope. Sam was a step away from a padded cell or Darth Vader, anyway. He needed someone stable, someone who, for example, didn't dream hop. And could look people in the eye.
Dean sighed.
"Don't make it sound like I want you hurt," Dean said. "I don't. I'm just looking out for Sam. Very, very VERY hurt Sam."