A sticking door is almost always caused by loose hinges, swollen wood from humidity, or paint buildup. Start with the simplest fix first.
Close the door slowly and watch and feel for where it binds. Mark the rubbing area with a pencil.
Tighten every hinge screw on both the door and the jamb side. A loose hinge causes the door to sag and rub at the latch side corner. This solves the problem more often than people expect.
Apply painter's tape around the marked area. Use a sanding block to sand down the rubbing spot working with the grain. Test frequently. For heavy sticking, use a Stanley Surform plane. Once the door swings freely, seal any bare wood with paint or primer.