It has to do with commitment he may not like the idea of sticking to one person which tends to happen with guys haha, sometimes they like the idea of friends with benifits so if someone asked if he had a girlfriend he could say no because your tecnically "friends". Also he could have been in a bad relationship and isn't sure he want to fully commit again. So if this question relates to you then you should sit down with him and give it to him straight that your not looking for a friends with benifits relationship you want the full thing. If you don't want the relationship tell him the same things but say It's not right to act like were together when were not.
Remember that what he wants out of a relationship is cuddling, hand holding, kissing, and presumably sex. If he can find these things without the negative aspects, why would he not take the opportunity?
He wants all the things that come with a relationship but no comittment. That way he can do whatever he wants with you, act all lovey dovey but can come and go as he please and be with other girls if he wants. You are letting him have his way with you. Cut him off ASAP and find a real man who will do these things with you and comitt