Donald Trump is mad at MSNBC's "Morning Joe." Again.

What set him off this time? It's hard to know for sure. But the presumptive GOP presidential nominee tweeted about his latest boycott Friday morning, shortly after co-host Mika Brzezinski shared her surprise — and apparent dismay — at the decision leading Republicans such as House Speaker Paul D. Ryan (R-Wis.) have made to reluctantly endorse Trump.

I think this is a different election; this is a different time in American history. You can't have it both ways. ... I just can't believe there isn't another candidate, at this point, that they would say, "You know what, I'd rather go down behind the right person than sell out America, my principles and everything I've worked for all my life to get behind somebody who I don't believe in."

Yeah, that might have had something to do with it. Recall that exactly four weeks ago, Trump started a Twitter fight with Joe Scarborough because he didn't appreciate the former Republican congressman's talk of an independent candidate. A few days later, he said he wouldn't appear on "Morning Joe" anymore.

Trump's holdout didn't last. Two weeks ago, he was back on the program, though the mood seemed a bit frosty. It's quite possible that we'll hear the real estate mogul's voice (just his voice, since he prefers to call in by phone) again on "Morning Joe" before long.

But it's worth noting what appears to be a trend: Trump seems to reserve his biggest media tantrums for the outlets, shows and journalists from whom he expects the friendliest coverage.

Brzezinski and Scarborough were — as they often remind their viewers — among the earliest believers in the viability of Trump's campaign. Early in the race, Brzezinski even made a wager with MSNBC contributor Mike Barnicle that required Barnicle to buy Brzezinski a pickup if Trump were to win the GOP nomination. Brzezinski has been gloating recently about winning the bet.

As Trump rose in the polls and started winning primaries, Brzezinski and Scarborough tried to argue that taking Trump and his base seriously was not the same as endorsing his candidacy. Not everyone was convinced. Even Trump seemed confused.

"You guys have been supporters, and I really appreciate it," he said on their show in February, after winning the New Hampshire primary. He amended his statement later in the interview, saying Brzezinski and Scarborough were "not necessarily supporters but, at least, believers."

Clearly Trump thought Brzezinski and Scarborough were on his side. Now, he seems to feel betrayed by their criticisms.

Similarly, an expectation of favorable coverage appeared to be at the heart of Trump's feud with Fox News during the Republican primary. Left-leaning outlets such as the Huffington Post were far more hostile (HuffPo put Trump stories in its entertainment section for a while), but he didn't seem surprised or troubled by their treatment. With Fox, however, Trump seemed to expect fawning coverage — and got frustrated when he didn't get it.

Trump's feud with Megyn Kelly epitomized that frustration. In an interview for the February issue of Vanity Fair, Kelly described Trump's effort to "woo" her.

He would send me press clippings about me that he would just sign "Donald Trump." And he called from time to time to compliment a segment. I didn't know why he was doing that. And then when he announced that he was running for president, it became more clear. But I can't be wooed. I was never going to love him, and I was never going to hate him.

The pattern is pretty obvious. Trump doesn't like critical news coverage, but he especially hates criticism from those he expects to love him.

This is the real reason why the GOP standard-bearer is angry with "Morning Joe" right now. Brzezinski didn't say anything on Friday that other commentators haven't said, too. But she's the one who is about to get a new set of wheels, thanks to Trump (assuming Barnicle pays up.) Where's the loyalty and gratitude?