I haven't run into this problem myself, met my husband in grade school, but it may apply to me making friends in general. The obvious connection for me is trust. If I love someone they will disappoint me like my alcoholic dad did. And since I wasn't raised in an atmosphere of love and trust I never learned it and don't know how to apply it correctly. When people don't live up to my idolized view of things I prefer to walk away rather than to stay entangled and flounder in an unfamiliar area. I have often wrestled with this.
But I have heard other recommend the John Powell books, Why Am I Afraid to Love?: Overcoming Rejection and Indifference.
http://www.amazon.com/Why-Afraid-Lov.../dp/0883473224
And here are a couple other links that looked interesting. Good hunting!
7 Reasons Most People Are Afraid of Love | Psychology Today let go & let it in: how to stop being afraid of love - positively present