Why "Central"

It is common to understand virtue as the balance or "just middle". In this perspective, when there is excess of a quality it is called "vice" and when there is fault, "defect".

For instance: If I am prudent, if there is an excess of prudency, then it will be a vice of distrust. If there is a fault, then it will be a risky hurry (imprudence).
An excess of pasion, for instance can be luxury (a vice) when it is an excess of sensuality. When there is a fault, for instance a husband doesn´t satisfy his wife as debt, then it is a legal fault, posible cause of cancellation of marriage.

Now, it is well known that there are no universal rules about this, unless you consider as "rules" those that change on time and circumstances. Because each individual has its own duty. It is said for instance "the duty of a king", but only to simplify: each king has a different duty, because he rules over a different kingdom.

Then, the "just middle" or balance is not the highest point of Gauss curve (the "normal" stadistical curve), but it depends on the position that a certain individual occupies in the universal concert.
That said, you can sustain the "just middle" as a definition of virtue.
In other words: you need to take proportion on account.
For instance: for a 100% pasional individual, the "just middle" is the extreme: 100% pasional. But for an individual with a combination of qualities in different proportions, that combination is his "just middle". The trick, then, is that we don´t have pre-defined proportions, but we have to find them in the way of the life.
Then there is the time factor, because reality is dinamic. So we can find our center; but in the next instant we lose it. In fact, we go on centering and un-centering, alternatively. That means "change".