Two words: unconditional love.
I'm dying to get a pup, but I live alone in a not-huge place, and I occasionally up and disappear for a few days with no warning.  I just don't feel like I could take care of a dog the way it deserves right now.  In reference to another thread, I would not green-card marry someone for $5,000, but I might marriage of convenience someone for dog-sharing purposes.