Changes in the weather can cause a door or the door frame to expand which results in the door becoming a bit too tight in the frame and sticking.
Sticking most commonly occurs in the Summer months when the humidity is high. The extra moisture in the air is absorbed by the wood of the door causing the door to swell or expand slightly in size. In the Winter months when the air is colder the humidity is lower. Less moisture in the air then means less moisture absorbed by the wood.
Sticking caused by weather changes is likely to be a temporary problem which will disappear when the weather becomes dryer.
Candle wax applied to the edge of the door which sticks can help reduce sticking in the meantime.
Loose hinges can also cause a door to stick.
When one or more of the door hinges is loose the result is a door hanging out of alignment with it's frame (called the jamb).
To correct loose hinges first try simply tightening all the screws with a screwdriver.
If there is a screw which does not tighten well when adjusted try removing that screw and sliding a small sliver of wood into the hole before replacing the screw and tightening again. The size of the sliver of wood will be a judgement call you will need to make and will depend upon the size of both the screw and the hole.
