Passion is a quality that English people fixate on too much. That's why they get so deluded when every international tournament rolls around, only to end up realising that passion is overrated and quality can actually get you places instead.
True, even Martial got criticised for just getting on with it and keeping concentrated after a missed chance, and Depay for not cheering much after he scored in Denmark. I think it's a general problem with fans that tend to focus on what they can see, and forget about all the stuff that is more important but not shown to them. Players are often judged to have qualities like fighting spirit just because they make a fist, act irritated or have a nasty facial expression. It means shit of course, fighting spirit shows itself in the fight. It's lovely for a dramatized match report, but that doesn't make it important.
After a goal, the camera switches to Van Gaal, and he's writing. Of course, that's a good time to write because the players celebrating the goal is not important to watch. The reason the camera is not on the players but on Van Gaal is the same why Van Gaal's eyes aren't on the players but on his paper: there's not much interesting to see on the pitch at that moment. So it looks like he's always writing.
Nontheless I think he should be more on the touchline. He's right about not beeing able to make real changes from the touchline, but sometimes the players wanted to rush the second goal, and wasted possession too much, or were too concerned about possession and lacked urgency. I think things like could be corrected by a shouting manager on the touchline.
Another reason is that it influences referees. I think English referees in general are likeable in their specific English approach of the game, but they tend to be easily intimidated by big name players and groups of players surrounding them, and managers getting angry by their decisions.
Also, as a minor point, it could help the chemistry between the players and the crowd. This hasn't always been there and even been negative, for example I can imagine that if the players are dwelling and passing sideways, and the manager spurs them on into more urgency, the crowd can join in with the manager instead of booing the dwelling.