He'd had enough.
The sophisticated lies, the cover-ups, the god-damned excuses that never really worked because too many people (too many people meaning Cuddy) knew the truth. For a while, he actually enjoyed playing into the delusion. He eagerly swallowed whatever House fed him. The rush energized him, kept him occupied.
Wilson savored keeping secrets, and that was fine, as long as they only involved him. A guy can't fall into three failed marriages without juggling a few lies on the side. But keeping secrets with House? That was...more complicated. Keeping secrets from Tritter? That was impossibly hellish.
House's addiction would ruin him someday. Wilson understood that, but as a friend, he refused to push the issue. House would, of course, fully acknowledge his pill-popping and not feel compelled to stop. He knew that Wilson didn't have the balls to force him.
But House's run-in with a cop provided the catalyst, and for the moment, Wilson felt relieved. Did Judas feel it? This lifting of the horrible weight from between his shoulders? This confession of guilt? This unburdening of the soul?
House needed rehab before he destroyed himself, and if it cost Wilson his friend, then the sacrifice was insubstantial.