What is it about Mozart's music that makes it so popular and enduring?

Of all Mozart operas, The Marriage of Figaro continues to be one of the most popular among audiences today.  What do you think it is about Mozart's music that makes it so popular and enduring more than 200 years after it was written?

Comments:

Complexity and believability.

Complexity and believability. Mozart's characters are real people with real problems shared universally with the rest of us. They are neither Wagnerian titans nor Donizetti's fluff balls. And Mozart's characters complex emotions and motives are reflected in, and commented on by the music. We can see ourselves in these operas.

And no matter how serious the subject there is someone with the frailty of Papagano who in many ways let's us laugh at our own foibles.