I've had friends like this too. Some people have just been broken down too many times to put their full trust in anyone. LoveIsOurWeapon has a point... just give it a bit of time:]
Some people have a hard time trusting others. Whomever your trying to help, give them time. It took me months to trust my best friend. Just make sure he/she knows you'll ALWAYS be there and you'll never judge them. You sound like an amazing person. (: