Why doesn't my friend stop tickling me?

Well I'm a guy and so's my friend, I'm 14 he's 13 and lately I've been wondering why he's tickling me all the time. He always does things like coming behind me and tickling me until can't breathe or telling me to stay still as long as I can and tickling the bottoms of my feet. I'm ok with tickling here and there but he's really tickling me all the time plus I'm really ticklish so it's getting annoying. I would tell him to stop but I don't know what to say to him about it. Anyone have any advice or know why he likes doing that so much?