Monolithium said:

I hope that Frank Oz has recorded new dialogue for this set. Because I desperately want to hear Midi-chlorians coming out of Yoda's mouth in TESB.

Yoda: Size matters not. Look at me. Judge me by my size, do you? Hmm? Hmm. And well you should not. For my ally is the Force, and a powerful ally it is. Midi-chlorians create it, makes it grow. Its energy is inside of us at the microscopic level and that in turn binds us. Luminous beings are we, not this crude matter. You must feel the Force inside of you; here, between you, me, and that's it because only a select few have it. We are the Chosen People, Luke.

Ahhhh. So much better.

While I’m no fan of midichlorians, I will say that the thing I got asked the most about as a kid when it came to Star Wars was why Luke could become a Jedi and Han could not?

I think Lucas may have been asked this question more than we realize and decided to come up with an explanation.