What does BUT mean?

It means he doesn't want the same kind of relationship. Anytime you're talking with someone and they say "It looks good...but" it means you won't be getting what you wanted. If it's work related, it means you will be re-doing it.

There could be a lot of 'but' reasons,...

... but I am not looking for a relationship.
... but I want to play the field.
... but I am moving out of the country.
... but I am getting a sex change,... ok, maybe a little extreme :)

Really there is no way to know. It's times like this that a question from you to him is the best way to go. Ask him what he means, what is the hold up.

Best of luck.

It means there's something in the way of how he feels towards you another girl or something that you or him are hung up on!