As your house gradually settles over time, small changes in the alignment of door jambs can cause doors to stick. Other things, such as loose hinges or new thick carpet can make it difficult to open or close a door. Fixing each of the things that causes a door to stick is easy, though, and requires at most a hand plane or circular saw.
When a door sticks in the jamb (the fixed frame around the door) look to see where the door hits the jamb. If it hits on the top of the door on the lock side of the door, shim the top hinge with cardboard to move the lock side of the door down a hair, away from the jamb. Remove the screws from one side of the hinge as in photo 1, cut a piece of thick cardboard (not corrugated) to the size of the mortise for the hinge, and reinstall the screws.
Photo 1- Shim hinge leaves to adjust the location of the door in the jamb.
Most of the weight of the door hangs off the top hinge, and occasionally this causes the screws to come loose, causing the door to hit the jamb on the vertical lock edge near the top. If you try to tighten the screws but they just spin in place, the wood around the screws is stripped. Try installing longer screws (2 inches) that will fix into solid wood behind the jamb as in photo 2.
In some cases the longer screws will not hit solid wood, but only plaster or empty space. In this case bore out 3/8 or ½ inch holes on the old stripped screw holes and glue in same sized sections of hardwood dowel as in photo 3. Use regular wood glue, yellow aliphatic resin, and let it dry for an hour or so. Predrill holes for the screws before installing them because the hardwood dowels are too dense to sink screws into without pilot holes.
If the screws are tight but the door still rubs on the vertical edge above the lock, you may be able to free it by deepening the top hinge mortises. Remove the hinge, and with a sharp chisel remove less than 1/16" from the bottom of each mortise. Don't remove too much, or you'll cause the door to bind on the jamb at the hinge.