What’s the deal with sundresses??

My (F) fiancé (M) goes absolutely nuts over sundresses. It seems like I’m completely irresistible to him when I’m wearing a sundress; it’s like turning on a light switch. He drops everything and is in like a trance or something. Not even him, but every other man I or my friends have dated is obsessed. So what is the deal with sundresses in particular?? I don’t get why they seem more sexy universally when ‘sexy’ tight clothing doesn’t get the same hype.