Getting cummed on

My boyfriend asks me every so often if he can cum on my face and/or in my mouth and for some reason i dont want him to. We've been together for a year and some change. He has been faithful and everything and he is such a sweet heart. He does so much for me and ive told him I'd do anything for him, but for some reason i just can't do it. Any help? Should i let him? Any reason why i shouldn't let him? Would me saying no to him all the time discourage him?