Tweek never liked solitude but was not accepted into another group. Craig wished for the solitude that the blonde had. He hoped for people to leave and the solitude he wished for only happened when the blonde was around.
Tweek loved when Craig was with him because it made him feel less lonely. So Craig started hanging with the blonde more than some of his friends like Token and Clyde. Though Craig wasn't in solitude like he wanted he found more peace in the blonde than anyone else. Tweek loved having Craig around. With Craig Tweek was relaxed and a little less paranoid around the pessimistic teen.
After a while of hanging the two started dating. For everyone else it didn't matter, though most thought they were dating, were not surprised or cared, except Cartman who did enjoy ripping on the two. Until Craig beat him up for his crude insults.
The main point of this that they learned sometimes solitude is a great thing.