Weaving, and related behaviours of head nodding/tossing, box walking and fence pacing are all about frustration, frustration because there is an obstacle blocking the path to the horse’s goal. That goal might be to get more room to exercise, many horses let rip with a kick and a buck after being cooped up. But it’s even more fun with friends.
Many stables don’t allow for physical contact between neighbours, and almost as many only allow a limited view of others such as only when the horse pops their head over the door. Research demonstrates that such living conditions are indeed a source of chronic stress.
Other research shows that horses choose to stay active and be out for longer when turned out in company. Since incidence and duration of weaving behaviour can be reduced by careful use of stable mirrors and/or windows to neighbouring stables and other surroundings (Reference), the social aspect of this frustration related behaviour is highly significant to the horse – hardly surprising since they are social animals ;-)
Mutually compatible group housing had the best effect on reducing stress in Yarnell’s research in this link.