I don't think a girl can become less attractive physically, if your in a conversation and enjoying yourselves!? I could see how she may become less attractive in your eyes if she had some views or habits that you didn't like i.e. opposite poilitical views or heavy smoker, but the initial spark of seing a girl that makes your heart skip a beat, is what drives us to approach.
You say you have high standards, so perhaps they're preventing you from fully enjoying the interaction with the girl as you feel she has to meet all of your criteria, for you to continue to see her as beautiful.
Another reason, may be that it's a defence against rejection. I've encountered a few guys, whilst I've been doing coaching, who go into sets without any AA, but eject soon after and when I say it seemed to be going well, as the girl was smiling and laughing, they replied that she wasn't "my type".
Guys will sometimes be so happy that they can approach and start a conversation with the girl that subconciously, they'll adopt a "quit while I'm ahead!" mentality, in as such as they don't want to have the negativity of a possible rejection, ruin the interaction when they ask the girl for her number, so they stay chatting and then talk themselves out of asking her for her details and just say that she wasn't worth asking!?!..
Just a thought!?...