Hard to imagine until you reach the point but it can be just a matter of boredom and sometimes a large tank can get just boring! So one way to perk up the old spirit is to try something new and perhaps more difficult. After a bit of learning the basics of how to keep fish alive, a big tank is so easy, compared to small that going back to small can really give much more opportunity to expand and do many things on a small scale rather than one single large scale item. I tend to like large fish and large tanks, so I do have a 120 but as I've settled into a less intense way of living, my favorite tank is a 20 long with a varied bunch of small fish.
I can go into the twenty and do an hours work and it looks totally different where if I spent the same time, effort and money on the 120, I would still need to "fix" the tank. And there is certainly much more challenge in keeping a small tank on the straight and narrow as they are so much more likely to become unstable, just due to size. A single dead guppy may pollute and kill a gallon tank where it would not even become an issue in 120?