Most Helpful Guy
The problem with telling a girl she's beautiful...is that you come off as desperate and patronizing.
Unless he is genuinely dating her...saying she is "beautiful" is usually gonna change the way she perceives him, for the worse.
That's why you don't hear too many guys approaching females of interest and saying she is beautiful...that only works in the movies! In most cases, the girl will think the guy is trying to hard to impress her, and her attraction to him will nosedive (whether she notices or not).