Maybe your favorite character died or left? Maybe a character died/was killed needlessly, or in some stupid way just appease a plot device? Maybe the storyline ran it's course, but they tried to drag it out in some cheesy pointless manner? Maybe the subject matter crossed some personal lines, and you didn't want to see anymore?
I lost interest with several shows recently because I feel they have gotten boring doing the same old thing after the main story had played out.