We have to assume when the house was built, the door worked fine. That means there has been settling which took the frame out of square. I saw a house where the second story doors on one aide were out like that -- and another one would close "on its own" because the settling from a new addition had taken the whole side of the house out of square.
Inspect the house for evidence of settling and the causes. You should deal with the causes ASAP.
If you are sure the house is now stable, I would use a power plane to shave the top of the door. Make sure the gap is even across the entire width of the door so the visual is consistent. Congatulate yourself for an easy fix and pray it does not come back!