system
sad: Took my dog to the vet today.  They had to put him down. 
human
I had to take my dog to the vet today
human
Oh no, is there something wrong with your dog?
human
Yes, he hasn't been feeling well. The vet had to put him down. ;(
human
I know words don't really help in this situation, but I'm so sorry. I've lost many pets and it rips your heart out. I'm sure you're dog had a great life with you if it's any consolation.