You fell in love with a game and it's characters, sunk hundreds, maybe even thousands of hours into it. It became a comforting, immensely satisfying part of your daily life. Then you heard a sequel was coming and got really hyped but when it came out it was utter rubbish...
Two of my favourite games of all time are Diablo 2 and Guild Wars.
Both of these games I was insanely hyped for the following games in the series and got them both on their respective releases days. Both were utterly disappointing crap when compared to their previous games and both probably contributed heavily to how I will now no longer get hyped for any game let alone buy one in their first year or two of release.