Why does your horse charge you when you approach him in the pasture?

What are his body signs? Is his ears laid back, or pricked forward? Does he appear tense or relaxed? Is the only horse you have? Did you call out to him before you entered the pasture or not? Did you have a feed bucket or hay bale on you at all? And, how long have you had him? It's most likely that he's just happy to see you and is running up to greet you, or thinks/sees you have a treat for him that he is super-eager to get from you.

If he's happy to see you, his ears will be pricked up and not laid back, he will not show the whites of his eyes like he's scared or challenging you (unless he's an appaloosa, which they always show the whites of their eyes no matter what mood their in), or appear tense. If his ears are laid back, he could either be in a bad mood, afraid of you, or scared you might hurt him and have learned to strike first. But, if he's a good nature horse (which I'm sure he is), then he'll just either be eager to get that yummy thing in your hand to eat, or he's just really happy to see you.