One reason may be that he doesn't feel like he needs to. He may feel secure in your relationship and doesn't feel the need to constantly tell you how much he loves you. Or, he may simply not be the type of person who expresses his emotions verbally. Another possibility is that he's afraid of being too complimentary. He may worry that if he tells you how great you are, you'll start to expect more from him than he's able to give. Or, he may be afraid that compliments will make you think you're better than him.