This expression developed from nine species of African parrots that are also called lovebirds. Lovebird mates are often considered to act particularly loving towards each other and like to stay close together.
It isn’t clear when this term first started being used to describe humans. Some sources put it as far back as the 1600s, but it was at least since the early 1800s.
In his novel Bleak House, Charles Dickens used the phrase.
