Guinea pigs are highly social animals for whom isolation causes distress and shortened lifespan; understanding their social needs is fundamental to responsible keeping.
The social needs of guinea pigs are so well-established that some countries have legislated minimum pair housing requirements. A lone guinea pig is a welfare concern equivalent to a lone dog — the species evolved for continuous social contact, and isolation denies a fundamental need that cannot be compensated by human interaction alone. Same-sex pairs with appropriate introduction are the minimum welfare standard; mixed-sex groups (with neutering) are preferred for social complexity.