As wood doors (and wall/ opening framing) begin to absorb moisture from the air (water vapor), the material expands. This cuases diffuculty when opening and closing the doors; particularly in warmer, humid weather.
It is generally a temporary issue, as swelling will go down in colder, dryer months (or if you choose to run your air conditioning system at that time). Additionally, you can choose to have the doors cut/ planed in order to account for the swelling (this should be done during the time that it's actually a problem).
Over time as a building foundation shifts in its footprint, walls/ openings tend to get out of plumb. This "racking", typically shifts door openings to the point that the door is hitting the frame. To correct this, a couple things can be done.
One option is to cut/ plane the door to the adjusted opening. Another option would be to remove the door and casing, then reinstall the door jams plumb (hang the door, install the casing, caulk, putty and touchup paint).
Loose Hinges will also give the door operating difficulty. Sometimes the screws get pulled out from forcing the door to close or open during a settling/swelling issue, and other times screws will pull out if the jamb is cracked which is a typical result when an installer doesn't pilot their screw holes.
These issues can usually, easily be fixed by using longer screws (if the problem is on the jamb side of the hinge). If it is on the door side, longer screws MIGHT work, but it holes MAY need to be filled and redrilled in order to fasten the hinge properly.