Mbloch's answer is a good one and it is quite thorough. However, I can see that some readers may infer a sense of "laziness" from that answer, so I wish to present the "opposite" view that, I think, shows a sense of consideration. I will demonstrate by actual example:
A while ago, one of my kids wanted a dog. I adamantly refused based on scheduling. The question I posed my kid - "Who will take care of the dog? Feed it and walk it, mainly?"
Why did I think this couldn't be done?
When the day begins, he rushes in the morning to get dressed, brush teeth, etc. get lunch and verify school supplies, with my supervision and he better be on that bus on time b/c I have to leave for work, too. No time to feed the dog or walk it, then.
Yeshiva day ends about 6 pm, and when he gets home, he needs an hour to clam down from the hectic day, eat, and do homework and get to bed so he's not tired to function the next day. And, frequently, his parents (wife and I, that is), get home late, ourselves.
In short - no one has time to feed or walk the dog, no matter what anyone intends. And what I mention is quite typical in religious households. Jewish religious daily activity is extremely busy and hectic. I won't even delve into what it's like for a woman or man (I do most of the cooking and Shabat shopping these days, instead of my wife) to prepare for Shabbat in the kitchen with Spot running around, barking or whining.
In actuality, if you had a dog, you would be mistreating, and neglecting it. So, avoiding the problem by not owning one is, in a way, a chessed.