It is really just preference. My preference is dogs, so all of my dream villagers happen to be dogs. For some, it is whichever villagers may fit in with their town theme, or if they simply like their design. Some people just pick villagers they like. The reasons vary; it is simply preference.
This explains it a little better than my post.
My dreamies are often based on three things:
1. Their design (and personality, to an extent).
2. Have I had them in a town before?
3. Their birthday's month.
I try to have as many personalities in my town as possible. In my current town that I'm currently creating, I have all personalities except for Peppy, because I find Peppy villagers annoying. On top of this, I make sure my dreamies have a birthday for as many of the months possible. I've also tried to include species I've never really had in many of my towns.
Vivian (Snooty, January 26)
Skye (Normal, March 24)
Phoebe (Uchi, April 22)
Jacques (Smug, June 22)
Erik (Lazy, July 27)
Boots (Jock, August 7)
Whitney (Snooty, September 17)
Elmer (Lazy, October 5)
Wolfgang (Cranky, November 25)
Carrie (Normal, December 5)
Including my own birthday, it covers 11 out of 12 months of the year so every year has at least one 'event' for the Town outside of holidays. It also has species I've never really had in any of my towns being Kangaroo, Alligator and Deer.
It's a pretty fun and unique way to pick and choose villagers.