My motivations are a mixture of nearly all of the reasons listed already, but I'll add something more: I enjoy building during the actual building process, but I also get tremendous enjoyment from thinking through future builds. If I have trouble sleeping, I'll mentally take a kit off of my shelf and build it step by step in my mind. If I'm stuck on a spot in a real build or daunted by a new technique I need to learn, I'll be thinking it through when I'm at work or on a long drive, etc. It's almost like meditating.