Why did Leonardo paint The Mona Lisa?

The Mona Lisa (begun 1503, completed 1506) is believed to be the portrait of a Florentine citizen's young third wife, Lisa Gherardini Giocondo. If that was the case, Leonardo likely either painted it because he was commissioned to paint it, or painted it simply because he chose to. There is no historical proof of the subject's identity, however, and therefore no way to confirm exactly why he painted it.

There is also a theory, however, that The Mona Lisa was a feminized portrait of himself. This is unproven.