I have a Cairn Terrier cross and I adore her, she makes my life better and happier. She has always been an easy dog: clean, quiet (she barely ever barks), had the shortest chewing phase as a puppy, does not dig, is completely non-aggressive, has no separation anxiety. We were incredibly lucky with her. She is, however, still a big responsibility and cost. For her first 7 years of life she only went to the vet for shots and spaying. But then she tore a ligament while playiong with another dog and needed 2 surgeries, and then developed a heart condition which needed expensive medication to manage. So we have probably spent over £5000 in her so far in the last 3 years.
Still, we are planning to get a puppy now, because we are dog people, and she is such a good dog that we hope she can help train the new puppy into good habits. Also, if she passed away and there were no other dog in the house I know I would find it extremely difficult to cope. We are very aware that things can go wrong, our dogs may not get along, and actually the puppy may change our dog's behaviour to the worse. We are very aware that we will likely not be as lucky with the new puppy as we were with our current dog in temperament and "easiness". I am taking time off work to spend with the new puppy to train her as best as possible to address any issues before they areise and I have booked a training session already, even before the pup arrives. And we are highly aware of the extra responsibilities and limitations having two dogs entail. Still, I do believe having this new dog will bring more positives than negatives. And even if I am wrong, it is a life being we are adopting, one who will be vulnerable, and dependent on us, so we will have to suck it up and love her even if it does not work out as well as we hoped.
I have always had dogs though, so I have an awareness of what is involved in owning one, and the love and pleasure they bring to your life, for me, compensates for any negative.