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...
4 and 5 didn't ruin anything for me. There's stuff I genuinely like about them that got me excited for the next game. Plenty I didn't like about them too.
Then there's Infinite... it feels like the DLC or post-game content to a game we never got. And the multiplayer was unplayable last I saw. It made me no longer excited for the next game.
I still do Halo game nights a couple times a year though.