It might be he's more seriously involved with someone else atm, so even though he might appreciate your potential as a romantic interest, if he's decided his loyalties lie elsewhere, that's what he's going to do.
Could be anything else, too, like having a hard time with something else that's come up and he can't process the rigors of romance until he's sorted stuff out. We're like that sometimes, and actually, you should hope this is the case, since when he gets over it he may decide he's ready to reciprocate with you, if your"much reason to believe" he liked you is legitimate. The not believing in friendship with romance potential part might just be an incidentally self-imposed rule for what he sees as an over-complicated interaction, so it might seem to blatantly fly in the face of context if you don't realize how differently his mind works.
I realize that may seem ridiculous and selfish and maybe even quite lame, but the fact is our heads are just in a different place. We tend to not have the automatic, allaround, even flow of navigating life neurotypicals have, especially when it comes to unconsciously putting together (coalescing, if you will) others' actions and then forming an unconscious reaction. We put the thoughts and feelings of others together manually, and have a hard time accounting what expectations might be.