There can be so many reasons why someone that claims to love you and want to share a future with you will run away. The fact that you don't know the answer means that he was not clear with you and left many unanswered questions which is terrible behaviour and heartbreaking because the mind can imagine every single scenario imaginable and yet what you need is honesty and truth no matter what it is. It will either clear the air and/or a misunderstanding or it will give you proper closure so that in time you can move on without questioning why.

It also depends on how long you have been together??

He obviously has fears and fears bring doubts. Fear of being honest with you and/or fear of his own feelings for you. Some men run from their feelings and need time out to figure it all out, the more you chase the further they run.

If you have tried talking to him and he doesn't respond then try writing to him. Ask him to be honest.

I sincerely hope he is man enough to do that and give you peace of mind at least. Stay strong