I recently had to put down my Cab even though he was happy (notice I didn't say healthy). It is still a hard subject to talk about but let's just say he had a parasite that we couldn't get rid of and it wouldn't have given him a good quality of life: He wouldn't have been able to have friends (quarantine for the rest of his life), medicine for the rest of his life, and very bad side effects from the medicine. To myself, my vet, and my husband, it wasn't right for Cab to have to live like that.
There's been times when we have had to put down a cow, a horse, a goat, a bird, a dog, a cat-it's always because the animal wasn't healthy and the animal just let us know it was time. It was a hard decision every time, but also what was best for the animal.
Good luck with your decision.