Did you ever stop to think that may be a good thing? I mean suppose the guy is trying to tell you something. Like say the girl he is comparing you to or saying you look a lot alike it is real obvious you're being compared to and told you look like someone that's hot, pretty, gorgeous, beautiful. Also maybe he's trying to give you the hint he likes you or turned on by you.
I don't get mad, but I do find it problematic. I disagree with the concept of all women being pitted against each other in a competition of looks :P I don't care if I'm better-looking, worse-looking, or similar-looking to the person I'm being compared to.
No. If you call another woman hot, pretty, gorgeous, beautiful, and whatever else, that's what you're calling HER. Not the girl you're comparing her too. When a guy says how great a girl looks and then tells his girlfriend or wife or whatever, "you look just like her," it's nothing but a major cheap shot to keep him out of the doghouse.
Competition. It'd be like a girl telling you that your best friend was nice and attractive and comparing you two.
Girls want to be the *only* girl a guy notices. So when he blatantly tells her he likes another girl, it undermines her security in him and makes her sad.
Girls don't like being compared. They don't like knowing there's competition. They don't like knowing that he could like someone else more. They don't see it as a compliment. They see the differences and think he must like those qualities (even if he didn't say he did) and try to be like the other girl so he'll still like her.