Oh, this has happened to me so many times.
Persona 3 FES: I mostly blame my perfectionism, trying to get maxed social links on my first run. The instant I fucked up, though, I just sort of stopped. I told myself "well, I don't NEED max social links, I can just enjoy the story", but my nagging completionist brain wouldn't let me. Probably will gear up for another go at some point, provided I have a Max Social Link guide that doesn't contradict itself and is way easier to follow than the one I found on GameFAQs.
Devil May Cry: I was getting a little bored of all the same-y castle environments, I guess. Graphics and gameplay haven't aged too well, either.
Tales of Graces f: I found the battle system frustrating as hell, and why everyone praises it as the best in the series, I'll never know. That, and while I know the point of it was to be as cliche as hell at the beginning, it was getting hard for me to force my way past that to get to the good bits where they start subverting and deconstructing everything (like basically every Tales game).
Metal Gear Solid Peace Walker: I always felt that 4 was the best place to stop, and playing this, I can hardly get invested. Just a case of prequelitis, I suppose. Who knows, maybe it gets better.
Advance Wars Days of Ruin: Got stuck on Chapter 10, found it way too hard, always ended up being a war of attrition where I was constantly bleeding money to make more units. I'd more often just give up and try starting over, to no avail.
Fire Emblem Shadow Dragon: Didn't hold my interest, plot was dull as hell, graphics were drab and ugly, gameplay didn't feel updated at all from 1990. Giving it another try, but all I want is Awakening right now.
The Legend of Zelda Skyward Sword: Not sure why, just kinda gave up. Admittedly, I wasn't feeling the motion controls and revisiting the same three dungeons bit, but I was still digging the game in general.
Xenoblade: Put it aside to focus on Xenogears, then come back to it later. Also got lost in like a dozen sidequests.
Sure there are more, too. Really should finish these someday.