Dogs can be suicidal and suffer from depression.
Although dogs are less depressed than humans and less suicidal than humans are the dogs can suffer depression and be suicidal.
I had a dog as a kid and he was depressed and eventually he stood there and gave me a sad look and was acting like he was saying goodbye and then he walked out in front of a semi truck that was going by.
I tried to stop him but I couldn't.
My dad told me that dogs can be suicidal and although they rarely act on it they do sometimes do it.
My dad saw it as well as he coming out the door.
He explained to me that the dog was sad, depressed and told me that the dog really did do the walking out in front of the truck intentionally.
It happens but rarely.
Some ways you can cheer up a sad dog are to give your dog a treat when they show a positive change in behavior. It might also help to set up a routine for your dog.
Feed them at a certain time, take them out for a daily walk, and have cuddle sessions in between; all this can help cheer your pup up.