People get tired of it, that's all. Especially if they played alot in the past (or worse, pushed themselves far longer than it was fun), it's easy to get burnt out and "hate" it. The thing to keep in mind is that ALL games are repetitive, and video games particularly so. The difference being that with most video games, you've had your fill after a few weeks or months, whereas MMORPGs try to continue to run for years. There are only so many ways to vary the gameplay, and after eight years on the treadmill some people are going to be fed up.

It's natural. Just learn to have a bit of perspective and it doesn't seem so bad.