forever together., a good enough reason? Options

[31miracles]

So I've been dating this girl for 15 months now (I'd say a averaged three year relationship due to the time we spend together). I know her as well as she knows herself and vice versa. We're really different. I go to a Ivy while she works full time at a restaurant to support herself. We are at the point where we might move in together for economic and geographic reasons, and we are talking more and more about how we will grow old together.

I'm scared. Commitment. Damn. Why do people get married?

My girlfriend I think is pretty, not Megan Fox status, not a great body, but a very cute face. I'm sure I could do better for myself (a prettier girl in college, educated - a future high paying job), but I'm at the point where I am honest, because I might be spending the rest of my life with my gf. My girlfriend has the best personality and loves me more than the world. I'm in college, and this is the time where I'm supposed to explore and meet different girls. However, I'm not big for causal relationships.

I've dated a few girls, and I know no one could ever love me as much as my current gf does. No one can ever come close. No one is as nice or considerate. Seriously, this girl finds me absolutely perfect and attractive. How the hell? I'm no where near perfect. She doesn't care about anything but me. We could be living in a box, and as long as she has me, she'd be happy. She has no doubts about me, but I can not say the same for her.

Is that a valid reason to spend our lives together forever? The fact that no one else would ever love/care for me as much as this girl does.

I love my gf, but I'm still young (18). I kind of what to explore, but my gf might be the best person for me and I'm too scared to risk it. If we are going to stay together, we have to move in together. I have to make a choice now. The pressure is on.