Some people are insecure, dependent or sensitive to rejection. They Usually are afraid to commit their love for others because deep down they fear their own emotional weaknesses. Do not pressurize her to commit, neither keep her telling that you love her very much as both can create equal pressure. Try to know her family background, her relationship with parents and relationship between parents themselves, probably you may find reasons. It’s wise to take professional help to resolve her insecurities for happy togetherness.