The main reason behind the difficulty behind closing or opening a door could be the door sagging. It can become highly problematic and embarrassing. But it takes very little effort to fix the problem and you do not have to resort to professional help to take care of it. Doors usually have three hinges. Take a look at the hinges. Tighten the screws on the hinges. Once the screws are tightened the door should not sag.
If the hinge is damaged then you will have to get a professional to come and replace it. It could also be caused by a cracked door frame. The cracked door frame would also require you to call a professional to come and shim the door to size.
Muscle power is good, but you sure can use it for better things, right?