You're not the one
Sally: He just met her... She's supposed to be his transitional person, she's not supposed to be the ONE. All this time I thought he didn't want to get married. But, the truth is, he didn't want to marry me. He didn't love me.
Harry: If you could take him back now, would you?
Sally: No. But why didn't he want to marry me? What's the matter with me?
Harry: Nothing.
Harry: If you could take him back now, would you?
Sally: No. But why didn't he want to marry me? What's the matter with me?
Harry: Nothing.
I've been in the same boat. You know, dating a person who is afraid of anything that is more than just a casual hook up. And I've been that person, too.
There many reasons why people refuse to commit -- bad experiences, there's someone else in the picture, not ready, etcetera, etcetera. Some of it may be real but 99% of the time, those are just bullshit because the truth is, YOU'RE JUST NOT "THE ONE". You're simply one of those girls Ted Mosby talks about in How I Met Your Mother. To put it bluntly, s/he only sticks with you until someone better comes along.

And odds are, one day, s/he will let go of those bad experiences, get rid of everyone else in the picture but one person, stick to one boy / girl and be ready for something real and probably a lifetime commitment. Regrettably, it will never be with you.
Okay, don't host a pity party in your bedroom. THERE IS NOTHING WRONG WITH YOU. For what it's worth, the person had an amazing time with you, that's for sure. But then again, YOU'RE NOT THE ONE.