"That dress looks great on you" or "Those are really cute earrings."
Why are women always complimenting each other on their looks?
Beyond the explanation that it's just a genuine compliment, perhaps the following forces are at work:
1. Reciprocity — The oldest trick in the book. When you tell someone she looks good, you're likely to get a compliment back.
2. Tribalism & Social Bonding — By complimenting someone, you're saying, "You're my friend. I like you."
3. Reaction Formation — This is the phenomenon where an anxiety-producing emotion ("I hate that she has a skinny body and I don't") gets replaced by its direct opposite ("I love your look today!"). Women feel threatened by attractive women. In this page of Jessica Alba bikini photos, the comments are telling: "She looks awesome. Gawd I hate her…" is just one.
Could it be that a good portion of the body image pressure women feel comes from the non-stop, public judging of other women on how they look?