This is especially important for rabbits who are home alone much of the time. The solitude of an empty apartment would be unnatural even for the wild animals whom we label as solitary. As she goes about her daily business, even a solitary animal like an orangutan is surrounded by other living creatures. Don't let a busy schedule give you the guilts about quality time your rabbit is missing. Think guinea pigs.
Some bunnies coexist contentedly with a feathered friend.
There may be somewhere a rabbit who is truly deeply solitary, but I have not met one in ten years of living with rabbits. There are some medical reasons for not adopting a second rabbit. Rabbits who are not altered will produce more rabbits, adding to the overpopulation problem. An unneutered rabbit who is too elderly for surgery, or a neutered individual who is frail or sick may not be able to handle the stress of an introduction to another rabbit, but may be a good candidate for a guinea pig friend. Two rabbits are generally not more expensive than one.