Have you got one of those interior doors that changes with the seasons? One day it closes fine, the next it’s stuck. It’s a common ailment. When the door was hung, it was painted or finished nicely all round, but chances are the top or bottom wasn’t finished. After all, no one sees that part of a door, right? Problem is, this allows the door to absorb moisture and dry out depending on the humidity, which causes swelling and warping. To fix the problem, wait till the door fits nicely, then take it down and seal all unfinished surfaces.
While you’re at it, you could spruce up the whole door. And believe it or not, there’s a proper technique. Paint the top edge first. Move on to any recessed panels, working from the top down. Then the rails (horizontal pieces), working top down. Then vertical stiles and edges. And finally, the bottom.
Direct exposure to sunlight can also cause problems with interior doors. In the winter, the sun travels lower in the sky and might hit doors it misses in summer. Consider blinds or window valances.
Loose hinges cause sticking, too. Attach the hinge with screws long enough to reach the stud behind the jamb. To ensure a really professional job, remove the door trim, and make sure you have shims directly behind the hinges.
Then there’s that door that just won’t stay open. It probably isn’t hung plumb. To correct this properly, remove the door and shim out the bottom hinge on the jamb, planing the door as necessary to ensure a proper fit. Or you can remove one of the hinge pins (if they are removeable) and bend it slightly with a hammer. This is treating the symptom rather than the cause, but it does the trick.