I've been cosplaying since 1999, and nowadays I'm very involved in helping run conventions, staff and judge masquerades, and teach workshops and classes.
What interests me most about costuming/cosplaying is actually the design and construction of complex elements. I love figuring out how to make things, and I love fine detail. Dressing up for a day or two is fun, but knowing that you figured out something tough and pulled it off really well stays with you for quite a while.
It's not unusual for me to be embroidering one costume for months at a time, so I usually only get a couple of costumes done per year.