Bunnies are social animals. They especially enjoy having a friend of their own species - for pet rabbits a male-female neutered pair usually works best. If you have a lonely rabbit, your local rescue or shelter may not only be able to find a friend for them, they may also provide a bonding (dating, marriage, bunnymoon) service whereby your rabbit can go meet possible suitors and come back to you a week later with their dream companion.
And don't think they'll love you any less once they have a companion - after all, you're still the one that brings all those tasty meals and piles of hay!
Size, age, breed - these things are not important to your average bunny looking for a friend. They know what they like and sometimes even the oddest looking pairings are the best!
Some bunnies, like their wild cousins, will also live happily in groups. The dynamics of this are far too complicated for mere humans to understand and therefore often difficult to arrange, but it does happen. If you are interested in giving a home to a group of rabbits, please let your local rescue or shelter know when you visit.