They get that way because they're small and they like you. When I'm carrying something (and don't want to trip and can't see where he is), I make sure to walk with my feet close to the ground and my legs close together. Kind of unnatural, but otherwise he's between my legs and under my feet.
It seems that many kittens do this, and if you accidentally kick him with your foot he will probably not be hurt by it, nor will he start to hate you. When I get dressed in the morning, my kitten runs around as if he were on fire, and often will go face first into my leg or shin. This morning he rammed into me so hard it actually hurt a bit! He just bounced off, shook it off, and continued the mad dash around the room. He figures I'm a moving obstacle, I guess.
Just remember that it's far better to accidentally toe or nudge your cat, who weighs very little, than step on them or trip and fall on top of them (since most owners weigh between 5-20x their cat's weight). The speed at which your legs move when walking aren't really enough to hurt a cat that's on its feet.