Why do people who stop being a couple, "break up"? It seems like it would make more sense to say "break down". Does anyone know why this is?
Break down and break up are two different phrases with slightly different meanings. although both may be used to within the context of relationships, they're used in different ways. Perhaps it will help if I explain the origins of the phrases.
This means to stop working, typically referring to a complex machine.
My car broke down on the M25 Motorway last night. It took me hours to get home
Sometimes we will use it to talk about a relationship stopping working too: mostly in this usage we are talking about the interaction, trust, love, and communication between the people breaking down. It usually implies that the relationship ending wasn't anyone's fault, and it was just the natural end of the relationship.
It does not always mean that the relationship ended instantly, and is more about the relationship problems, although it usually does indicate a break up which may or may not come later.
It describes the process of a relationship ending, not the actual event.
After I lost my job my first marriage broke down.
It will tend to be used about more serious, adult relationships - you are more likely to hear about a marriage breaking down, rather than the relationship between a teen couple.
This is the act of a relationship ending, and is the equivalent of a marriage divorce.
It comes from a more literal phrase 'to break up', meaning to physically separate into smaller pieces.
The spaceship entered the atmosphere and began to break up
This is used in a more literal sense than 'break down' in that it is talking about two people in one couple (larger 'object') separating into two smaller, separate objects (themselves).
Me and my boyfriend broke up when I saw him kissing Suzy after prom.
It may be used about both serious, mature, and less serious relationships, but is more often used regarding younger relationships.
"to break up" when referring to the relationship of a couple is a kind of metaphor, an image of comparison. When you break up a nut, the two parts of the shell come apart and are no longer connected. The "up" in this use comes near "apart/asunder".
A non-authoritative answer may be because of how one breaks, for example, spaghetti. The ends go down (usually) and the "break" goes "up".