"Hate" is a loose term here. I always start watching a reality show because I enjoy it. However, as time goes, things usually take a turn a season or two later - personalities change, shows experience a cast shake up, producers start giving someone else the bad edit, cast members start to act differently because they're more aware of the camera, or someone makes the news for bad behavior. It's all about the snark.
Heck, competitions are great for hate-watching too. The competition starts off with amazing competitors but someone gets too cocky, or a judge gets rid of a fan favorite, and there's all sorts of conspiracy theories that abound.