I think Mattis wants to see us out of Syria as well. With only an armchair analyst guess, there must be something behind the scenes that had Mattis worried from the strategic perspective regarding Trump simply announcing, "We're done, ISIS is finished". Much like when Obama said the "JV Team" was finished. From the news at least, it sounds like Mattis was very worried, to the point of pretty much just showing up at the White House for one last plea.
There are many ways to pull back. We saw that Obama did it wrong. My guess is that Mattis didn't want Trump repeating that error.
On the tangent, it has been kinda funny to watch both the Obama cheerleaders and the Trump cheerleaders pretty much defending or criticizing the opposite of what they were doing when "the other guy" did it. Suddenly Dem leadership are all acting like Mattis is their bestest buddy, and ISIS is a great danger and we need our troops in the ME. I've also seen way too many conservatives saying some pretty stupid things about Mattis in the "good riddance" sense (not so much in Congress, but more the "conservative pundits").
The last thing I want to see with Trump (or any President) is to be surrounded by yes men. I was pretty impressed with Trump's original cabinet choices and what looked like him surrounding himself with experts that he would listen to. Sadly, I'm starting to see that go in the opposite direction. He still does have around a year to turn that back around though.
I don't mind Trump doing the "I know more than anyone else" stuff with economic issues. In many ways, he has shown himself to be right. I'm scared about him doing that with national security issues. I'm very curious about who gave him information and advice, and what that was, that would cause him to blow off Mattis.