Marriage?

For Tamagotchi, it is the method of reproduction and getting a new egg, essentially restarting the life cycle. In some versions, this is more elaborate than others. For example, in the Tamagotchi M!x, marriage allows one to mix the traits of the parents. For other tamagotchis, this is barely noticeable and serves as the 'gateway' to the next generation. (Like the Tamagotchi Tama-go, or 4U). Marriage is not required in any of the tamagotchis (I think), but the tamagotchi may die from old age if unmarried (and if it is an earlier version tama). If you have tamagotchis other than Tamagotchi M!x/Meets, it's likely a minor feature. (The parent does not affect offspring in most other releases.)