Men know that most women want to have an emotional connection with someone before they sleep with them. Men know that a lot of women think it's romantic to be friends first, and then the friendship blossoms into a relationship. Men know that they have to jump through all these hoops first, before they can get laid. And that's really all romance and courtship is to a man: hoops he has to jump through to get laid.

Men pretend to be “just a friend” at first, even though they want to sleep with you from day one. Otherwise they wouldn't be spending any time, money or attention on you, because these are limited resources and they need these resources to attract a mate. They can't afford to squander them. So they apply these resources to the female that looks to be their best bet to get laid. But they also know that they can't tell the woman on day one that they want to sleep with her, because she'd think it's creepy. So they play along with the illusion that it's “just a friendship” that “suddenly” developed into more, when the woman finally feels inclined to sleep with the guy “because they have a deep connection.” But that was really his goal from day one.