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I'm curious WHY qualifying a girl always works/has her trying to proove herself.
Is it a big no no to tell a girl your horny/thinking about her in a sexual way?

a girl trying to prove herself is qualifying, how would asking a girl to qualify, constantly have her trying to qualify?, was that your question?
qualification is more or less just a tool, by asking a girl questions about qualities she posses that you desire, you can get to know her and find out what about her it is you admire and show interest for her demonstrating that she can provide that value for you, not only that but it gives you indications of how comfortable/attracted to you she is,
if a girl is not willing to answer a simple qualifier that requires little to no investment then it is safe to assume you have little to no attraction with her, if she will sexually qualify herself to you, it shows she is pretty comfortable/attracted to you, chances are laying a chick who is trying to brag to you, ''YEAH I GIVE SUCH GOOD HEAD'', will not be as hard to lay as a girl who say, ''WTF IM NOT ANSWERING THAT FOR YOU''
as for stating your desires, depends on the girl/situation, it is fairly irrelevant, if it comes off congruent most people will put up with that as long as they just assume it is congruent to the way you are but it would probably be wise to cover your social bases, doing something like that can end up getting you blown out (but it can also end up getting you blown)
i often just test waters to see what i can get away with and how far i can push a girls comfort zone verbally/physically, it is probably alot better to over escalate then calibrate it down (possibly get blown out), then to under escalate and get to be friends,
but it is not as if im an expert, i just try to bang girls
thats my 2cents