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Check your hinge pin bushings. With the door slightly ajar, lift up and down and check for movement. If that's not the problem, and it gets to be more than annoying, readjust the door on the hinge.
My driver's door has always been hard to close since I got the car in 1985. I took it to the dealer (under warranty) but they couldn't fix it.
I recently looked at the door striker and latch and they don't seem to be adjustable. There is a wear mark on the top of the latch opening, so the latch and striker definitely don't line up correctly. I did notice that the rub strip on the door is a little lower than the one in front of the rear tire, so I think it's a door alignment issue.
Based on the above, I think zr1fred's advice fits this situation. My door does not have up and down movement, so it looks like I need to adjust the hinges.