The origins of the elven race or even when they fostered their desire to serve are lost to history, but the earliest cases of house-elf servitude under wizards date back to the days of the Roman Empire, where several emperors kept house-elf servants. House-elves are quite long-lived, with their life expectancy being around 200, and in addition to being fantastic home-carers and cooks, are excellent wine-makers; elven wine is some of the most coveted beverages in the entire wizarding world.
House-elves breed infrequently and never without their master's permission. House-elves do take a mate, although once again, never without their master's permission. These requests are hardly ever denied; wizards have a vested interest in the procreation of their servants, as it means more elves to help around the house.