Guys, why would you constantly make eye contact, shows signs of interest,...

...yet not talk to a girl? By signs of interest I mean: friends pushing you into them, eye contact 2-3 times a day, somehow ending up near them in the hallway. If someone (me) is returning the signals, why wouldn't you start up a conversation, especially if you see them regularly throughout the day? This has been happening since April, not including the summer which ended in August. Honestly, I'm positive he's interested, but I'm shy too. His closest friends are the most confident people in our grade...if that means anything.