Thanks, guys. This is really helpful

A few writers have told me before to show the characters' responses to something, emotionally. "How does she feel about this?" I always was skeptical about it: let's say the MC just heard the news that his child has diabetes. I mean, isn't it obvious how he must feel? So the suggestion that I take out internal dialogue is advice that I like, but am hesitant to do. Yes, I know that's subjective, but how do you guys respond to it?
I have trouble writing longer novels. This one is about 140 pages, double-spaced. It's YA, so I can get away with that length, but I'm afraid if I cut too much I'll be left with something too short to be called a novel. I'm not sure why it is I have trouble writing longer books; I don't seem to know how to stretch the plot while maintaining the tension.
I have experience loving novels that had lots of scenes which were wonderfully descriptive and said a lot about the uniqueness of the setting, but didn't really have relevance to the plot. It makes me wonder why you never hear writers encourage that method.