We feel it is important for kittens under six months to have a cat buddy. So we adopt kittens in pairs or into homes with existing cats. You'll find that most rescues have this same policy, though you can sometimes find a shelter that will adopt a single kitten.
Kittens need interaction with other kittens for healthy social development.
A kitten learns a lot in the first several months of life from its mother and littermates. Isolating a kitten from other cats can delay the kitten’s development emotionally, socially, and sometimes physically. Kittens who remain with one of their littermates or another young welcoming cat companion tend to be healthier, happier, and better socialized pets than those who are raised alone.
Even loving, caring, humans are not adequate substitutes for kitten companionship.
Even when a person is fortunate to be home many hours of the day, the amount of attention a lone kitten will demand is likely to occupy more time than the person has available. A pair of kittens will still want to interact with people, but can keep each other occupied. Most cats, regardless of their age, are highly sociable and are truly happier living with other cat companions. This in turn makes them better pets.
Kittens are curious and crave constant stimulation.
Out of boredom, a single kitten will often entertain itself by chewing plants, climbing drapes and furniture, unrolling toilet paper, exploring electrical cords and sockets, etc. Kittens who live with other kittens may sometimes do these things too, but with another kitten to tumble around with, it is less likely they’ll need to entertain themselves with destructive and dangerous behaviors.
Kittens bite and wrestle with one another--this behavior is normal.
You can't prevent a kitten from doing what comes naturally, any more than you can force a two-year-old toddler to sit still. Though it's not acceptable for a kitten to bite and wrestle with its human companions, it will do exactly this in the absence of a companion to play with. Even if you can tolerate this behavior from a kitten, it will not be so cute when you have an ill-behaved adult on your hands.