Rabbits are companion animals and in the wild they live in large groups, enjoying plenty of social interaction (and love - you know the expression!)
As pets, they really should be kept in pairs (as a minimum) and mixed sex tends to work best - of course, neutering is recommended if you don't want your pair to become a fluffle! Neutering can also have health benefits and can prevent tumours in females and aggression in males.
So, if your bunny didn't come with their some-bunny (generally under 12 weeks of age, they pair up quickly) or has sadly lost them later in life, what do you do? It is not as simple as just buying a new rabbit - they have to bond and this can be a slow process. Many rescue centres have expertise in bonding bunnies so they are a good place to start. If you're going to do it yourself, be patient . . .
Rabbits can be surprising vicious and so the best place to start is with separate enclosures where they can smell/see each other and this can gradually move closer as they get more accustomed.
Once they seem fairly friendly (and this could take weeks - don't lose heart), you can try to introduce them - this should be in a neutral space and you must never leave them alone. Keep a thick towel handy in case you need to separate them quickly and intervene at the first sign of tension. Keep repeating this visit, gradually building up the time, until they are relaxed in each others company. Feeding them together can help.
Once they are happily grooming and snuggling together, you can move them in together and let the bunny love begin!
There's lots of helpful advice on bunny bonding online:
The animal charity Wood Green also runs free e-learning courses for bunny owners: https://woodgreen.org.uk/workshops/rabbit-elearning-course