We always had dogs when I was growing up, but here's one reason I would never have one again:
Take a walk around the streets where you live, dog fouling everywhere and Wandsworth Council does *nothing* about it. When you take it out can you guarantee you're going to pick up every turd, and leave it as clean as it was before your dog got there? The answer is no, because you aren't going out with one of those Parisian pooper-scooper-scooters, and you will miss it when your dog is out of sight sometimes.
You may be the most responsible dog owner in the world, but you walking your dog around the streets sends a message to everyone else, who may not be as responsible, that this is normal behaviour. But dogs and built-up areas, especially densely populated cities like London, just don't mix.
Even the most ardent dog lover in the world does not want to tread in your dog's turds, so how do you imagine the rest of the world feels?