What a good question..
I think for me (someone with kids at home, airforce/housewife) I've always loved animals. When I was about 4 my Mom finally convinced me that I couldn't have an elephant for a pet and that pets had to be small. I must have really taken this to heart, because I have had a soft spot for small dogs ever since. When I had my first pet of my own, my husband and I were pretty broke and couldn't afford to spoil him like we would have wanted. Now with Sandy and Tia we are in a better place financially and are actually able to do more for them, so we do. That's the practical answer.
The emotional answer is that after I had my daughter I was told I couldn't have any more children. I had wanted a big family. After grieving for awhile I began to research dog breeds and now I have Sandy and Tia, my 3rd and 4th children. I really treat them as such, they are a huge part of our family and we make decisions based on them as well as everyone else. I spoil my children with toys and clothes, so why not the furbabies too?
So that's my long, drawn out reasons for why I spend way too much money on my doggies
Oh yeah, and I really love shopping too