Pigs are herd animals and it is best that they have one of their own kind for companionship. It has no effect on their relationship with you. But it gives them a buddy to sleep with, argue with, and just be with. It increases the quality of their life greatly.  If you already have a pig and decide to add another there are a few things to consider.

First, what is the size and attitude of your pig? If large and aggressive, he will be very rough on the newcomer. If large and shy he will still fight but won't be as rough. In general, a young pig is the best choice to add as it is not as threatening to the resident pig and pigs will not hurt a youngster like they will another adult pig.

Almost all meetings between strange pigs result in a fight. Usually it is harmless, a few scratches down their sides and then they become friends.  Occasionally they will rip an ear or a lip. Pigs with large tusks should not be mixed without having the tusks trimmed first. It doesn't seem to make much difference whether introducing pigs of the same or opposite sex (providing that the males are all neutered; never introduce two un-neutered males). The least fighting will occur if the pigs can be put together in an area over which neither has claim;
a neutral space. After they have been together for a day or two, then move them back to the regular living area. Do not introduce them INSIDE the house if your pig has been living there already.  The yard is a much better choice as they get to know one another.  One pig will end up being "boss" and from then on there will be no more fuss.

Many of a pig’s woes can be cured by the addition of another pig. It is a natural condition for them. People, dogs and cats are fine, but another pig is a true kindred spirit. They will sleep together, pal around together and fuss with each other, getting many emotional needs filled that we can't fill for them any other way.

If your new pig and the original pig refuse to sleep together at first, give them time. Pigs are territorial and their sleeping area is their most valuable space.  They may need many months before they feel comfortable enough to share their most vulnerable and private space. Plan to accommodate separate sleeping areas if necessary.

Pigs are wonderful, funny, and inquisitive. They create bonds with humans that defy reason. Giving them a place to roam and grow and basic food, water and shelter will make for a well adjusted, happy pig. Add a few belly rubs and you'll have a friend for life.