If a guy talks to me at all out of nowhere, or pays special attention to me at all, I assume he's interested. A guy usually don't start talking to and hanging out with a girl because he needs more friends in his life. He does it because he's attracted to her and sees her as a possible significant other.
So, any girl that "friend zones" a guy probably knows that he's interested, but she's afraid of telling him she doesn't want to date him, so she makes up the excuse of "Oh, but I just see you as a friend...."
Sometimes (and I'm not saying any of you guys are like this), a guy gets a certain thrill out of being the victim. He derives pleasure from being friend-zoned because it gives him a challenge and the thrill of a chase. He thrives on his intense emotions of loving her so much but not being able to have her. He is at her beck and call when they're friends, but if she does start to date him, he loses interest because the thrill of pursuing her is gone. This is usually found in E-type guys.