The younger they are, the more they prefer to be with others. It helps them feel more secure. I've raised them from nickle-size up and notice they tend to become more independent around quarter to half-dollar size. An adult is perfectly fine by themselves and probably prefers it that way with the exception of a mate.
Edit:
I also notice that temperaments vary, just like any other cichlid. Of the 10 nickle-sized I grew out, one was a bully to the other angels in the tank (the biggest one). The rest were very peaceful. Of the 4 that I kept (two pairs split between two tanks) one of them tried to assert himself to the male Bolivian Ram in there, but once the Ram got settled in, he turned the tables. No biting or anything, just posturing. I'm sure once the Angelfish gets fully grown, the Ram may think different though, hehe.
In the other tank, the two angelfish go in the Ram's cave and eat with them. They all hang out together with no aggression. It's completely different than my 55G that has more floor space, which is odd.