It's considerate to compliment someone's hairdo.
Sam is walking into work and notices his coworker has also just arrived with a completely new hairstyle.
Sam wants to greet his coworker.
Sam greets her and compliments on the way she has done her hair.
Sam doesn't acknowledge the change in appearance and simply says hello to her.
Sam's coworker feels encouraged because someone likes her new hairstyle.
Sam's coworker is disappointed thinking that he didn't notice the new hairstyle.