On a well-built shed, this shouldn't be a problem for a long time. Doors stick or sag when the building shifts, usually from an uneven foundation settling over time. The fix starts underneath: make sure the building is level. If it's shifted, releveling the base often solves the door issue.
For minor sticking, check the hinges. Tighten any loose screws, and lubricate the hinges with a dry lubricant spray. If a door has warped slightly from moisture, sanding the sticking edge usually takes care of it. On our buildings, the doors are hung on solid frames, so most issues trace back to the base rather than the door itself.



