Not sure about who it's most commonly used about.
Well, my teacher made it sound as though the person was showing strong outward signs of whatever the word with ～がる is, so that's why I said be antsy. For instance, うれしがっています makes me think of someone who can't stop smiling ear to ear, and 恥ずかしがっている makes me think of someone who has turned red in the face and is shying away from making eye contact or even facing anyone. That's the impression I get about it.