I lived just outside of Seattle from the age of 10-19, then moved to SLC and started going to U of U from there. I am 23 now and still live in SLC.
The main reason that I moved was that I began to dislike the gloomy and rainy weather in Seattle. That was a big factor. I didn't like the mild summers where it never gets too hot, and the mild winters where it doesn't really get cold or snow, except in the mountains.
I didn't want to go to UW because I don't like living in a big city like Seattle. I like having a house with a garage for my stuff, nice backyard, and close proximity to the mountains. Seattle didn't have the lifestyle I wanted. The population density is too high for me.
I didn't want to go to Western mainly because I was an engineering major, but even other than that, the gloomy weather is even worse up there. And the drives to skiing were still really far.
I came to SLC and it took some time to get settled and find a good niche, but once I did I loved it.
I like the fact that the seasons are very different. It is really hot and never rains in the summer, so you have lots of time for outdoor activities. You don't need to plan according to the weather. I rode motocross at the 2 MX tracks 15-30 min away all the time, and started skydiving with the 2 dropzones and a wind tunnel within 30 minutes.
I love the campus for Materials Science and Engineering that I am just finishing up studying. I get paid to work on and develop lots of cool cutting edge research in the awesome labs and tools I have access to.
Housing is a lot cheaper in SLC compared to Seattle, but that also comes with a factor that most jobs around here pay less than in Seattle.
As with most decisions, it depends on the person. For me, I will honestly never move back to the northwest. There were things I liked about it, but that doesn't outweigh the things that I find most important. I see myself definitely staying here in SLC for at least a few more years. The only places I think I would move to would probably be CO or CA.