...What is the motivation of the mainstream media to be against Donald Trump?
The MSM is establishment and Trump is not. The establishment, virtually in any walk of life, looks dimly at outsiders in general. They don't like change.
There are clearly political motivations, as the MSM is 90% or so liberal.
And of course there is ratings/money motivation. If their network or entity asks the most combative question, that will get the social media buzzing and bring attention to them.
Personal too. They don't like him personally and many libs have a superior complex, and think they are better than him and they feel they have to show this moral supremacy when and where they can.
Lastly but certainly not least, it is there job to confront politicians in power.
Ideally though, the MSM shouldn't be "against" nor "for" Trump as they should be gathering info and reporting and letting the viewer decide for themselves. But those days are long gone.
confronting Trump vs. being against him. 
"Lastly but certainly not least, it is there job to confront politicians in power."
For example, political reasons. The MSM HATED the fact that Trump wouldn't share his tax returns. They also HATE the idea of a wall. It is obvious and it shows.
But "confronting" him with facts when he is factually incorrect, is their job.
incredulity that he broke recent precedent in not sharing his tax returns, and that building a 'beautiful wall,' would solve anything!
-- Modified on 2/21/2017 11:56:22 AM
Like all true narcissists, he takes being confronted on what he says or does as being against him!
...Thank's J.D. for a sincere response, my question for you is how did mainstream media get taken over by liberals ? & who is paying for it, after all MSM is a for profit enterprise, as is Donald J. Trump.
There are clearly political motivations, as the MSM is 90% or so liberal.
And of course there is ratings/money motivation. If their network or entity asks the most combative question, that will get the social media buzzing and bring attention to them.
Personal too. They don't like him personally and many libs have a superior complex, and think they are better than him and they feel they have to show this moral supremacy when and where they can.
Lastly but certainly not least, it is there job to confront politicians in power.
Ideally though, the MSM shouldn't be "against" nor "for" Trump as they should be gathering info and reporting and letting the viewer decide for themselves. But those days are long gone.
-- Modified on 2/21/2017 4:24:20 PM
And, the press is responding, whether they are aware of it or not, as one would respond to a malignant narcissist. When all the leaks from disenchanted in his inner circle come to light, and the dust settles, you will see that I'm right.
http://medical-dictionary.thefreedictionary.com/Malignant+narcissism
Trump is a piece of shit. He's woefully inept, incredibly insecure, ill-tempered, and really just dumb as fuck. He's not equipped to be president. He's really not equipped to manage a 7-11. In the short time he's been in office, he's flubbed and misstepped numerous times in ways that simply can't be ignored; and he takes incredible umbrage when asked about those missteps. Those are the facts, and aside from editorial pieces, all I've read from the "mainstream media" is an accurate recording of those facts. Anyone who would couch that as some sort of Trump grudge is infected with a nasty strain of anti-intellectualism.
So the premise of your question is flawed (unless you're challenging his dumb ass supporters to explain what possible reason the media would have to be against him).
Which trump did to the media. From the beginning with this birther thing, how he uncovered this and that which at first was cute. But all lie or stretch the truth and the media accepts some to a degree but trump took it to another level. So once the media started to get "wise", trump figured the jig is up..hence now the media is the enemy.
To make this " so-called " President squirm and squeal while ramming the truth down his throat and up his ass..