You know... I don't think I'm either really. Maybe I'm a third new category, someone who really hates dating (or is really lucky). I had one serious relationship in high school which lasted a year, and after that was finished demolishing itself, I didn't date again for two years. Then I met Eric, got engaged to him after three months, stayed engaged for another 3 years, and married him. So I never casually dated OR had to be in a relationship at all times.
Thus said, my best guess on commitment phobic people is that they haven't met the right person for them yet, or that their priorities are in a very different place from yours (and so they're not the right person for you).
Although I do think relationships take a lot of work to maintain, I do believe that love is something you either know you're in or you don't, pretty quickly, and that you don't have to figure it out. You just have to figure out if you love someone enough not to kill them when they're being incredibly annoying.