Do guinea pigs need a cage mate? What happens if you don't have one?

I've had my guinea pig since late last June, and I love him so much, so I want him to be happy. He seems okay alone. He isn't depressed or anything, he popcorns and runs around having fun. He eats and drinks too. We called a pet store early on and they said guinea pigs were territorial and getting another was a bad idea, but I see so many people saying you need two or more, I'm so confused and need some help