Why Are Lemons Sour?

There's the obvious answer--lots of acid--but what do they need so much acid for? As far as I can tell, it seems that the lemon is no natural phenomenon, but was created when ancient peoples bred sour oranges with citrons.

The true origin of the lemon is apparently shrouded in mystery; in the 1st century B.C., a Near Eastern priest was subjected to a great flinging of citrus, and there is controversy over whether the fruits in question were lemons or citrons. For many centuries (apparently), lemon trees were mostly grown as ornamental fixtures.

In the 18th century, James Lind discovered that lemon juice helped fight scurvy in sailors (Vitamin C is also known as ascorbic acid, and more acidity makes for more sour taste). At some point in this timeline, bath salts come into the equation, and then the popularization of lemonade. In short, though, lemons are sour because that's the way people want them to be.