Can be anything from bad experiences you've had in-game to losing the cartridge to problems resulting from being a fan. I'll start.
When I was little, around the time Ruby and Sapphire were released, my father came to my mother's house and took my sister and I out for the day. On the way back we stopped by a Toys 'R' Us store and he bought me both games. I started with Ruby (Groudon was my personal favorite out of the two) and as I just got through Team Magma's hideout my cousin asked to borrow it. My mother made me begrudgingly hand over my game ("you have the other one just play that and let her use yours for a bit") so I made do with Sapphire for the next few weeks. As those weeks went by I went to collect my game, but several things were amiss.
1) She used the Master Ball on a Pelipper.
2) Half my good Pokemon were gone. My Swampert, who was my mother fucking G was gone.
And the kicker:
3) She'd somehow managed to fuck up the cartridge to the point where it would physically cause your GBA to overheat. I tried playing it on my GBA but couldn't get more than 10 minutes in without the system dying. I did this one too many times and as a result, my black Gameboy Advance died.
When Fire Red and Leaf Green were released, my mother got me a GBA SP (which I still have and guard with my life) but I'll never forget the day I lost my GBA.