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...
Halo 4, kinda suck tbh. This is coming from someone who play the MMC so i basically marathon it and is able to compare it back to back, and it peaked at Reach. The gun play is wonky and no dual wield, Covenant somehow become the bad guy again after the event in 3, and none of the one that help human defeat Gravemind came back as an ally.
But it doesn't ruin the franchise for me though, to me canonically there's only 5 Halo game. The rest is fan fic.