There are a million different answers to your question.
I had a well paying, senior position back home, in a large company, with all the perks. However, it played havoc with my family life and personal life.
I was overseas for 2 weeks every month, going to multiple countries, staying in great hotels. All my mates used to tell me what a great job I had, especially during the economic downturn. I am highly specialized and experienced in my field. Even with the huge layoffs companies were going through, I was bulletproof.
People thought I was insane to resign and come to Taiwan with my wife and teach English (I still dabble a bit in my industry).
For me it was a lifestyle choice. I was constantly tired and stressed out. I used to suck down pills to make me sleep, and then suck down a few more to wake me up.
I had no personal life, or time to do the things I enjoy as I was basically working seven days a week. On the weekends and evenings the Blackberry did not stop beeping, and I was expected to answer due to my position. It was f*%ked.
Now, I work 2 hours in the AM, come home have lunch, walk the dogs, do another 2 hours in the afternoon, come home and chill for a few hours, and then go do a few hours work in the evening.
I have weekends free, can spend time with my wife, do my outdoor hobbies on the weekend, chill at home in front of the TV at night, read books. Stress level is down to zero now.
Sure, I make a fraction of what I did at home, but to me it is worth it.
Well, thats my reason. I am sure others have plenty different ones.