The mutual enemy for all doors is moisture. When a damp door dries, it can become warped. If it is always in a damp atmosphere, there is risk for rot settling in. The minimum maintenance of an outside door is to protect the upper and lower edges with priming oil. It is these end parts that absorb moisture most easily.
When painting an inside door, it is worth removing it from its hinges and placing it on stable blocks so that it be easier to scrape and paint the frame.