Most people enjoy a number of solitary activities, with their level of preference for solitude over companionship being determined largely by their mood. People who prefer solitary activities as a matter of course are often referred to as introverts, whereas those who dislike solitude and prefer to be in the presence of others are called extroverts.
To be "in solitary" is to be punished by Solitary confinement.
Some animals live a mostly solitary life, only looking for others in mating season, e.g. feral cats and bears