You're around 18 years old, right? So, I assume your GF is also around that age. She probably is aware that her male friend likes her (think, as a male, would YOU act as he does around a girl you DIDN'T like?) but isn't taking it too seriously. Yet. She probably just likes the attention. Watch out, tho, sometimes that flirting can lead to stronger interest and is a common, ages old tactic used by young men to "get in under the radar" of the ingenue. Take action, as suggested above. The "askmen.com" site is surprisingly on the ball in these matters, IMO.