Everyone has different perceptions of beauty.
In ancient Greece, emphasis was placed on the body, in particular the male body, and everyone was proud to show theirs. In ancient China they bound women's feet because they believed that small feet were beautiful. In Burma they placed rings around a girl's neck, her collarbones collapse, her neck becomes deformed and the weight of the rings is terrible but she... she is beautiful. In America women slowly kill themselves, place themselves under the knives of strangers and are never happy; they want for a beauty which is an imperfect ideal.
Himawari found him aesthetically pleasing. It may seem shallow of her but given her condition she really had no other measure of others. It was only through his own obstinacy that she had actually gotten past outward appearances and smalltalk. Though a popular girl, Himawari kept everyone at a distance. It was a subtle action, so subtle that nobody had ever noticed. If they had, it would have been a guarantee that the same rumor-mongers that had destroyed Watanuki's reputation would have eaten her alive. Metaphorically speaking that is.
Yuuko found his soul amazing, even in her line of work. There are many things that make a person, but the core component is always a soul. No matter how a person is, there is evidence of their actions on their soul: the good, the bad, the ugly. It is the compounded record of gifts and punishments of a person's reincarnations, their ideas and their beliefs. Trials become the hammer and anvil of the gods and Watanuki had had many trials indeed. Watanuki has one of the purest souls she's ever seen, even if she includes Zashiki-warashi.
Shizuka found his mind and heart extraordinary. Kohane may have called him a beautiful person when they met (Watanuki, of course, had vehemently denied this and exclaimed "Where?!" emphatically), but Shizuka thought that Watanuki had a more beautiful heart. No matter the condition of the soul (though that did influence a person), the mind and heart are the result of hard work and dedication, and he was of an opinion that such dedication and determination made a person beautiful, or at least set them on the course toward perfection.
They can't help but think that others can't compare to him in any way.
He has become their measure in all things.
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A/N: If someone could tell me what chapters are about Watanuki giving up his memory I'd be much obliged. Much thanks to everyone.
+Andy