I've only had this happen twice, but this morning my mom will be going to the vet to have my brother's pet rabbit put to death, because it is ill. For me, a pet is just a pet, and nothing more. It is something that can be replaced, unlike e.g. grandparents. Therefore, I am usually at a loss what to say to people who have lost their pet (are going to lose it, like my brother) and are really sad about it. I can understand that some people form emotional attachments to their pets, but since I have never experienced such a thing, I am usually at a loss for words when people tell me they are down because they lost their pet.
Things like 'my condolences' seem too much. But just saying 'Oh, I'm sorry you that it makes you feel this bad' sounds like too little to me. Are there any nice responses to the announcement that people have lost a pet or are going to lose one?
Note: Since some people are interested in an age aspect, let's just forget about the old, lonely people for a while. I am looking for something I can use for little brothers and friends, so anywhere between 14-35 years old.