For cats that are left on their own for long periods each day, it is a good idea to consider providing another cat for company. Sociable interaction with another cat can greatly enrich their daily lives.
The younger they are when introduced, the greater the chances of their getting on amicably most of the time. After about two years of age, acceptance of another cat can be a bit random, but they will very rarely totally reject another cat in the long-term. After a time even those cats that do not become great friends can still learn to tolerate and live with each other by keeping to their own territory.
Getting litter mates gives you the best chances of a pair getting on. Where other cats are introduced it is preferable to do so with kittens. The ‘curiosity factor’ can be used to advantage by placing the newcomer in a room to themselves with litter tray, bed and food and water bowls, for two days before introducing the resident cat to them, especially if the resident is older than about six months.This gives the new cat a place to hide, plus some of its smell will be on you and in the room, giving it confidence. This will also allow the resident cat to gradually get a bit used to the newcomer's smell as well.
Bringing two older cats together, particularly where one has been resident for some time, may lead in some cases to some hissing and fighting at first. However, tolerance can increase with time and by using such methods as: feeding them progressively closer together, rubbing them alternately with the same unwashed towel to transfer their smells between each other, using cat pheromones or other medications in the short-term (please talk to your vet for details) can all help.