This question contains major plot and ending spoilers. Seriously, don't read any more if you plan to ever play this game.
When you find Clementine at the end of the game, it turns out that the person she's been talking to was the owner of the car from which you choose to either take the supplies or leave them. In my play-through, I took them. Now, the only reason he gives for everything he's done is that you took his supplies. So what happens if you don't? Is the stranger another person altogether? Does he give another reason for stalking you?