He's always touching me , why ?

My best friend is always touching me , messing with me , bugging me. For example ill be sitting with him and he'll be on the laptop ill be watching tv and he'll just start bothering me like poking me , tickling me and oh grabbing the back of my neck. We have talked about things that well excite the other in a relationship and he knows all of mine and I know most of his spots. I asked him to stop he said sure but the next day he did it again and said I made no promises with this big old smirk.He's the only one I allow that close cause i dont like people hands near my face or anything and yet when he does it it doesnt bug me. he's always giving me nuggies and biting me not hard just playful bites sometimes he'll grab "them" it used to bother me cause iv never let any guy get close...im...kinda shy of people.I draw a bunch im a inspiring artist. he's learning how to draw from me some now and one day we were really bored so he started drawing all over me. it was.....relaxing very relaxing. he knew i was nervous specily when he drew on my tummy and stuff and more ( iv been friends with him for more then five years so iv seen him in his undies and vise versa so there really isnt any shame between us as far as body wise ) Im a bit confused though because he sais im his sister and yet he does all this stuff and people think we're going out. He always sais no and always sais im like a little sister to him but why does he always go for my neck and shoulders. he's been biting my shoulders and around my neck. and he knows it like turns me on bad and stuff. iv talked to him and i have told him I do care for him more then just a friend but he said only sister. Im confused because its only recently he's been really really flirting hard and stuff and as soon as i flirt back and do the same to him he seems to back off a bit. He has a girl friend ( long distence been going out nine months from imvu they talk on the cell and stuff ) he sais he loves her and wants to see her one day. but why would he act like that taword me if he likes this other girl. Ummm help ? Should I flirt back more , iv concidered just kissing him and leaving it at that for him to make the decision if he wants to be friends , more then friends or stop all together. honestly.....he's driving me a lil nutty. I love him I do I know him iv known him since I was around fourteen. im twenty now. has anyone else delt with something like this ? he waits for my other friend to leave the room sometimes before he really starts well in all words teasing me. im just bout ready to just sit on him and start biting up his neck and shoulders and poking him back just as evily as he does it to me. he does get " happy" all the time around me but i dont read into cause guys get "happy" all the time over small things too.