There are three possibilities of why she may be hesitant.

1) Another guy involved, she may be torn having feelings for two guys, and doesn't want it to go any farther cause she doesn't want to eventually hurt you.

2) Low self esteem, she may feel that she cannot live up to your expectations, and doesn't want to dissapoint you.

3) (Least likely) maybe a religious or belief system implication involved.

Sounds like you really like her though, the problem is, there is something that is not being said here. True love requires two things.
Complete honesty and full disclosure. Make sure you are getting the full story.

All in all, she's prolly just playing a teasing game with you, enjoy it.