I sit for multiple reasons. For me it is more about the dogs and their owners than the money. I am lucky enough that we don't need the income from sitting. When we moved from Seattle to San Jose I gave up my full time job, rather than find one down here I have taken classes, do "crafty" things, take care of the house and hang out with my dog. These are all well and good, but aren't the best replacement for the community of family and friends that I left behind.
I find that through Rover I get to hang out with a variety of dogs and meet some awesome people. As Laura said, the guest dogs help keep me motivated to go out and play, they have helped my dog feel more comfortable around groups of dogs (she was always good one on one, but was shy and would hide between my legs if multiple dogs tried to say "hi" at once), sharing her toys is no longer an issue, she has lots of playmates and most important (if you ask my husband) is that it stops me from getting a second dog at this point (though I do get to compare and contrast what I like and don't like about all of the breeds I sit for).
Also, I found Rover as an owner looking for a sitter. I wanted an alternative to boarding. We had done the traditional kennel, where we had to pay more for her to be played with, as well as a place that was more like an off leash dog park, where I could watch my dog sit in the corner on their camera. She hated both of those, so Rover was a wonderful find.
Bottom line, I really like being able to give people the experience that I want my dog to get.
For myself, I am at a point in life where I want to have kids, but I am not financially stable enough to do so. So I have dogs as a replacement, but my fiancé says we can't have any more.. So I dog sit instead!! It keeps me happy and him happy. The money is nice too.