Now call me clueless, but for the longest time, I never knew all this. When I had an interest in a guy it would be all "why did he do that?" or "is he just being nice, because he's generally a nice guy?" or the typical "does he really like me"? Luckily I've gotten smarter (or did my research).
To get to all these conclusions, I've simply asked my guy friends straight up. A couple months ago, I was talking to one of my guy friends and I complained about guys being confusing, with which he replied, "Guys are easy. We only have a two class classification system." Curious and intrigued, I demanded he tell me more.
Basically he went on to say that girls get stamps on their heads whether they like it or not. It's either "fuckable" or "non fuckable." Either you get thrown into the accept or the reject category. Accept meaning I would bone her and reject meaning no, there's no possibility in hell.
Of course there are exceptions to this rule, and just because one guy wants to bone you, another may not. Or vice versa.
But hey, don't take my word for it. Here's a real deal testimonial that pretty much sums up my discovery:
"I'm a guy, I know what we think. I know how we scheme to get in the pants of girls we like, regardless if they are attached to someone else or not. It's what we do. We think with our dicks most of the time. If you hang out with a girl long enough 'as friends' it's almost guaranteed at some point you'll get laid if you try. We are a shady bunch, and most of us have schemed, lied, or manipulated to have sex with someone who otherwise would not have."
Can't say I'm surprised.