Why do women blow hot and cold?

I have a 'friend' who's been unspokenly teetering on being more than a friend for some time, but here's the rub; some days she'll initiate a bit of contact - tease me to playfight, poke me with a foot till I fondle a leg, shove her head into my arms to be nuzzled and stroked, etc; then she'll turn up next time and act all platonic, outright avoiding opportuntines for any such fun. It drives me up the wall, so why does she do it?