My horse is best alone. She came from the track so was used to lots of horses. She was supposed to be a companion for a gelding I had. He died. I got other horses over the years, but never had more than 2 at a time. Now she is alone and I swore that if I saw anything that I suspected indicated any kind of loneliness I would get her some kind of companion. But, she seems to be loving it on her own. I have zero problems with her behavior.
Sometimes, there are horses about 1/4 mile away. She'll look at them, then just do her own regular thing. Horses can go by on the road or be in the driveway and she'll look interested for maybe 10 minutes, then go off again.
Depends on the horse, the environment and the care.