Alternatively, over time, your interests or attitudes about something evolve, and what used to be awesome, is now not really something you care enough about to be burning your mental bandwidth on it.
And finally, well, sometimes you just find you don't gel with a person the way you once did, assuming you ever really did. Maybe you haven't actually spoken to them in months, and/or when you do, you just find yourself at odds over things. It's a natural human process, there's nothing wrong with that. You can't be besties with everybody.
Thus, every once in a while, it becomes time to engage in a bit of discreet unfollowing. I tend to devote a week or two to the process, going through my morning feeds, very carefully watching my LJ, Twitter, Tumblr, and Pinterest pages, and for each item saying, "Did this item/poster make my day better by being there? If this feed or person had not posted, would I miss it? Most pressing of all, did seeing this thing actually make me less happy than I would be if I hadn't seen it?"
If the net result of those questions isn't "This is clearly a valuable person/thing to continue watching!" then it's unfollow time.
Also, while I'm at it, I should mention that if you don't get any value from reading my posts/tweets/whatever, feel free to discreetly unfollow me as well. I won't take it personal. ;)