For some reason as soon as I get to the “end” of the game, I completely lose motivation to play. This happened with DOOM: Eternal, Sekiro, Bloodborne, Paper Mario TTYD, Cassette Beasts, Shovel Knight, Oblivion, Baldur’s Gate, God of War, Mass Effect… Pretty much every game I’ve ever played. It’s like the fun part was the journey, and I’ve seen everything the game has to offer, and I just don’t really enjoy the ending process. Feels like a formality at that point.
Same. I have a thing where I like to complete everything i can in a game before beating the final boss. I discovered the Master Quest, and then nothing would do but to beat that first and roll up to Ganon a god. Bashed my head against that wall for a while then put it down. Cue the second half of your first paragraph.