Your doors are expanding and contracting. This condition is typical, and is a result of the level of moisture in the air. When the air is damp, the doors tend to expand and may rub in several locations. When the weather is hot and dry, the doors shrink. Another cause for this condition is house shifting caused by changes in the level of moisture in the soil.

There are two major precautions you can take to prevent this problem from occurring. First, paint or seal the door on all six sides. That is, when painting the front and backside of a door, don't forget to hit the top, bottom, hinge side and latch side. This will protect all exposed surfaces of the door. Second, the crawl space below the house should be ventilated to keep the soil dry. You should also make sure the soil surrounding the foundation is sloped so water runs away from the house. And don't over water your lawn or landscaping. All of these precautions will help minimize the house from shifting and, in turn, help prevent your doors from sticking.