When a door sticks, there can be more than one cause. The sticking could be the result of humidity, which has swelled the door and made it slightly too big for the opening. It could also be the result of movements in the framing that have thrown the jamb out of square. A third cause, which is easy to diagnose, is that the paint on the door hasn’t dried completely and is sticking to the jamb. Each condition has a simple remedy, but completely fixing the problem may involve taking down the door.
Grasp the door handle and, with the door open, push it against the jamb and pull it out again. It shouldn't move; if it does, it means the hinges are loose. Tighten them with a screwdriver. If any screw doesn't tighten, take it out, pack the hole with wooden matchsticks and drive it back in.
Close the door after making sure the hinges are tight, and examine the gap between the door and the jamb. Identify any place where it touches with a light pencil mark on the door and on the jamb.
Drive a 2-inch screw into the jamb on the mark to draw the jamb closer to the framing if the door sticks near the bottom. Sink the screw head below the surface of the wood and cover it with wood filler. Sand the filler with 150-grit sandpaper and touch it up with paint.
Take down the door and lay it flat on a pair of sawhorses to correct sticking near the top. Plane the edge of the door around the mark with a hand plane to remove about 1/16 inch of wood. You can also do this by sanding the edge with a belt sander. Set the door back on the hinges, test it, then paint the part you planed or sanded.
Check the edge of the door and the jamb for flecks of loose paint if you can't find any other reason for the door to stick. The paint on either the door or jamb may be tacky. Rub soap or petroleum jelly on the edge of the door to correct this problem.