To pay tribute to the character, to have fun making the costume and dressing up. I love getting to dress up as some of my favorite characters. I've never thought of it as "sexifying" the character... some characters are made to look sexy to begin with, so maybe that's what you (and other men) assume women are trying to do when they cosplay. Cosplaying is a lot different from what people do at Halloween.
To pay tribute to the chara. And because it's hella fun, hahah. My next cosplay will be Lara Croft, because I personally think she is an amazing character, well developed and because I can. (: I used to do theatre a few years back, so it's in my blood anyway, Lol.
My last was an anime character... hm. Yeah, cosplaying is awesome. And acting as well.
i did a pool. it was defiantly not that sexy lol. though i did it so i could shout "i want you in me haru!" which is this guy from a swimming anime called free. i've never actually seen the anime, but the parody of it 50% off is so hilarious i had to do it! XD