User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-50.11.220.20-20130603211353/@comment-3146930-20130603213036

A lot of the time, that only works on monsters with a big ego. More animalistic monsters (Hydra, Nemean Lion, and gryphons for example) wouldn't care if you called them pretty. The reason it worked on Arachne is because of her pride and Annabeth knew exactly what buttons to push as she has the same flaw. Note how Annabeth didn't really complement Arachne herself, as that wouldn't have worked at all. She complemented Arachne's work, which was the key to distracting Arachne.

You also have to look at monsters in general. They can't be killed forever and know that they will return sooner or later. Demigods on the other hand will eventually die so monsters see them as weak and see themselves as unstoppable. The Gigantes are another good example, as they know they can only be killed by a god and a demigod, and with Olympus closed they aren't above toying with their opponents, which has cost at least 5 of them their lives and one was forced to retreat, as he toyed too long