Why must music critics review what they obviously hate?

I think it'd be nice if music reviewers were designated with material they either enjoyed or were at least curious about and didn't despise. With increasing frequency it seems, I'm reading album and concert reviews from critics whom obviously despise the band and/or genre they're working with, and the final result comes across like a Fox News segment on President Obama. Once in a while, the authors despise the music or concert experience so much, it makes for a somewhat humorous review. However, even a blind person can see the critic's biases, that he or she likely had a migraine from the first to final track of the album or concert, and it'd probably be in everyone's best interests (including the bands themselves) if they didn't have to write the review in the first place. That's all. This is probably the shortest blog I've ever written, yet I still felt the need to write it for whatever reason. Go me!