Here's exactly why Donald Trump finally answered a question about Stormy Daniels

Here's the exchange between Trump and reporters on Air Force One:
Reporter: "Did you know about the $130,000 payment to Stormy Daniels?"
Trump: "No."
    Reporter: "Why did Michael Cohen make it?"
    Trump: "You have to ask Michael Cohen. Michael Cohen is my attorney you'll have to ask him."
    Reporter: "Do you know where he got the money to make that payment?"
    Trump: "No, I don't know."
    It's not a lot. But, when you consider that Trump hadn't said a word about Daniels and the money paid to her in exchange for her silence by Cohen, it's a significant advancement in the story. Trump is now on record as denying knowing about the payment. That's a big deal.
    Daniels' lawyer, Michael Avenatti, quickly seized on the comments -- tweeting, "We very much look forward to testing the truthfulness of Mr. Trump's feigned lack of knowledge concerning the $130k payment as stated on Air Force One. As history teaches us, it is one thing to deceive the press and quite another to do so under oath."
    And it's hard to imagine that Trump's lawyers were particularly pleased to see him wading into the storm of Stormy. (I'll show myself out.)
    So why did he do it?
    My experience in covering Trump is that the simplest answer is almost always the right one. And the simplest answer is this: Trump was in a good mood. He was feeling his oats. So he figured he would pop back into the main cabin and chat with reporters for a few minutes. He likely didn't even think about what sort of questions he would be asked.
    Consider the context: Trump had just spent more than an hour at an event in West Virginia ostensibly designed to tout the tax cut law. Having watched the whole thing, I can report back that it was a total and complete love-fest for the President. There were a dozen or so people on the dais with him -- ranging from the governor of the state to regular folks -- and each one of them praised him to the moon.
    Trump was clearly thrilled, sitting with a big smile on his face as others touted what he had done for them. When he spoke, the crowd laughed at all his jokes and applauded loudly when he was done.
    In short, Trump was among friends. (He won West Virginia by 43 points in 2-016 -- one of this largest margins in the country.) And he loved every minute of it.
    The event ends. Trump gets back on Air Force One. He's feeling pretty, pretty good about things. And, despite all of his attacks on the media, Trump likes to interact with the media; he likes the back-and-forth. So, he heads back.
    Now. Once the Stormy Daniels question gets asked -- and then followed up (and followed up again) -- my guess is that Trump rapidly reconsidered his decision.
    But he was trapped. Unlike in the Oval Office or on the White House grounds, feigning that you can't hear the questions being shouted at you by the media isn't really an option in the close quarters of an airplane.
    The result: A comment -- albeit brief -- on Stormy Daniels. Which, if I know anything about Trump, turned a very good day for him into a much less enjoyable one.