What do you do if a guy who is your best friend decides he loves you but you don't like him that way?

tell him completely what you think about him/her as this will in-fact improve your relation as well as its clarity. it is good to tell him that you don't like him that way as it wouldn't hurt him that much. but he would get more hurt if he'll come to know from some other person what you actually think of him. trust me things will go well and good and if he is your best friend he will understand your feelings completely.