Get some grease and make sure all of the tracks are lubed up properly. If the window track design isn't that great, friction will cause forces to act in the wrong directions and can make the windows come off track. I'm not sure if that is what the problem is for your car, but that is a common problem for many car models. I would imagine that whatever is happening to your car is only happening on the drivers side, because it has the most use and wear, and that it wasn't happening when the car was new. So something has changed since it was new, and if nothing is bent, broken, missing, etc., the problem is coming from some sort of wear and tear or a friction problem.

So next time you have the door panel off, make sure to grease everything up nicely and see if that helps.