When we take a pet, we promise to care for it and love it for the rest of its life.
I disagree, slightly. While I don't see pets as disposable, I don't think that you have to keep one forever if it is a bad or highly problematic one that isn't worth the effort to your family.
There are so very very many wonderful animals who perish in shelters every single day, hundreds just in your immediate vicinity, why should one of those die while you nurture an animal that doesn't work for your family because you happened to choose it first?
I plan on keeping pets for life, but there are definite dealbreakers. The highest on the dealbreaker list would be harming other family members (humans especially, but including other pets). If you have a dog you love and adore who ends up being child aggressive, do you make your life a constant struggle trying to juggle the incompatible animal around your child? I feel basically the same way, to a lesser degree, about our pets. I won't keep an animal that is a danger or menace to others. There are too many animals who have a lot to gain over the one who is jeopardizing it. Does that make sense? I think it is an issue with a gray area, not black & white.
We have two rescue dogs, and I frequently take dogs in that need new homes and locate one for them. Once we rescued a dog from the shelter who ended up having some very major issues. I even contacted the local dog-whisperer and she basically told me that it would be over a year of intense training and the outcome would still be iffy on whether I could break the dog of her 'habits'. That dog went back to the shelter and opened up a spot in our home for the very wonderful pyr mix we adopted next, who was scheduled to be euthanized the same day. I can't see that as being wrong for our family, I see it as more fair for a more deserving animal who can make our lives happier. (It wasn't easy, I cried the entire way bringing her back, but keeping that first dog was not the right thing for my family as a whole)