Why do guys blow you off/act unaffected after a REALLY fun time?

I came with my friend to see her "friend," well turns out his roommate was there, and while my friend was busy making out we talked for hours,... Show More

Updates:
so I told his roommate to give him a message for me: that he can call me anytime...and now I haven't heard a word. BTW he definantly found me attractive. Why do guys do this? Have a great time and act unaffected?