Well, when I think back on the reasons for all the times I've left schools there's a lot of variation and this doesn't include those I eliminated through a phone call, viewing or taking a trial class. I just had a couple of people ask me to tell them about my background so I've already written it up. For those who're interested in ~ 27 years of off and on training and why none of it has lasted forever (yet):
Iwama Aikido - 2 years. Quit when I dropped out of college and moved to a different state.
Fencing - 2 years. Quit when I dropped out of college and moved to a different state.
If I'd stayed in the area I expect I'd still be fencing. I might still be doing Aikido, it's a little hard to say.
Fencing was what I was passionate about at the time, but I couldn't find a fencing school after I moved to Washington (poor timing on my part - a world class school fully opened up about a year later but I'd given up looking until years later). There was a ton of Aikido in Seattle but I wasn't excited by anything I could attend with my work schedule, etc. And so...
Tracy's Kenpo and Tai Chi (with martial applications) - 9 months to a year??
I started Kenpo in Washington because a friend of mine who moved here about the same time had been doing a different flavor of Kenpo before he moved and loved it. We joined a Tracy's School together. I left because the kenpo instructor we'd been working with quit and I wasn't very happy with his replacement, plus my work schedule had changed and made it a lot harder to get to class, but mainly, the instruction wasn't what it had been. Knowing what I know now I'd have stayed for the Tai Chi, it was great, but I wanted something harder at the time.
Hapkido - about 5 years. School closed for financial reasons so I had no choice. I loved this place and would probably still be there if it hadn't closed.
Yoshinkan Aikido - 6-9 months. It wasn't really what I was looking for at the time. I felt like it wasn't as fun as the larger, more flowing Aikido I'd done previously and yet wasn't as practical and effective as the Hapkido I'd just had to quit. I might have stayed with this longer if the head instructor taught more of the classes. He was quite good, but he traveled a lot for work and his #1 student that taught in his absence was..... not what I was looking for.
About this time MMA was taking over a lot of mind share in the martial arts world and it felt like all the commercial schools were either MMA/BJJ schools, kid oriented, or Tai Chi for health and I wasn't interested in MMA/BJJ at that time and I've never been interested in Tai Chi sans applications. I did about a decade of more off than on garage training in a number of arts including:
Historical Fencing (Rapier) - 3 months? Good school. I could afford the classes but not the equipment (.com bust) and it wasn't quite what I was looking for at the time so I didn't really WANT to figure out how to pay for the equipment.
Kali and Silat - 6-9 months? Instructor was having health issues and it was a long drive through rush hour traffic to make class. I was also trying to start my business at this time. The combo made it hard to continue.
Sport fencing (again) - ~ 3 months? - This was 12 or maybe 14 years after I'd quit fencing the first time. I found that it didn't inspire me the way it had the first time and I was SO bad at it in comparison to where I was when I quit that it was a real let down. I think a lot of it was that the school had a lot of students trying to compete at the national level and then hobbyists that were completely ho hum about things. I didn't really mesh with either group. I guess if I'd stuck with it at that time I might have been pretty good just in time to start competing in the old person divisions, but that in itself was kind of demotivating at the time... Now that I AM old, well, I might give it another try at some point.
Boxing for ~3 months of private lessons - I wanted the skills but wasn't interested in participating in boxing as a sport, the instructor wanted to teach it exactly like he'd learned it and was only interested in it as a sport (and strictly as boxing, not even as a component of MMA). He was a pretty good teacher but we weren't a good match.
~ 3 months - A really great combo of Muay Thai, mixed grappling (influenced by JJJ, Judo, wrestling, etc.), boxing and bits and pieces of other traditional martial arts. The instructor's life situation changed and he stopped teaching out of a garage and started teaching strictly Muay Thai out of an MMA gym. I couldn't easily make the new place and time fit into my schedule and I wasn't particularly interested in that school's program. I also kept expecting the instructor to pick the garage training back up, but it hasn't happened yet.
Aikido (yet a different style, the name escapes me at the moment) for ~6 months. This was a really good instructor. As it was not commercial training it was tough to get access to mats and a safe and appropriate place to practice. I also took a job on top of my business during this time and was working 60-70 hours a week so I just sort of burned out.
So, here I am, it's been about 4 years since I last trained in anything and a lot longer than that since I've done enough of anything to get good at it. I'm looking at schools and trying to find something that's a good match for me. It's complicated by the fact that I'm not sure how long I'll be in Seattle, but I'm currently guessing at least another couple of years so I want to make the best of them.