... most probably because in some cases we want to share the happiness of our moments with other people and sometimes this forum is the only place or preferred place. Asking people what does he mean, gives us an excuse to tell people about it and hear their opinions, just as we would tell our best friends in life. It is always nice to have someone to share our joy.
The other alternative is that for some reason we have been really poor at reading these signs before or we are lacking in confidence and hearing other people agree with us or assure us gives us the courage to take the next step.
No matter how many issues of Cosmopolitan they read or how many "101 Things About Men" articles they study religiously, a lot of women like that will never get a clue. It's big business, too, sadly enough. So many women are so preoccupied with understanding tiny details that they forget to see the whole picture, and people make money off of it. Sometimes, men wonder as well, but I hardly ever see them obsessing the way girls do.
Don't get why girls on the question have to be all rude about it but I think often women just want validation of something they have been wondering about or are just insecure about the situation and want a outsiders opinion. Not everyone can be smart when it comes to love so we need to respect those questions as well. It may seem obvious to us but not them. Sometimes people are just genuinely confused about something and it doesn't kill you to just answer them. This is after all a Q&A website.