If there's a group dedicated to growing that plant, like roses or orchids, they're probably a member. Which makes the craziness even more intense. A dozen beloved African violets on the window sill will swiftly multiply to fill the extra bedroom and maybe half of the garage, happily blooming under artificial lights.
Bonsai enthusiasts start out with a single plant that captures their attention.
Soon they build a shade structure in the back yard for more plants and cancel vacation plans because they can't trust someone else to take care of the bonsai. I recall one couple who returned from a trip to find that their prized bonzai plants had been stolen. Perhaps it's a color we find irritating. I know a gardener in the Napa Valley who dreads the thought that any of the thousands of tulips he plants each year will come up with red blossoms. His employer abhors red flowers.