I hate wasting time and I hate wasting my student's time. I want every hour in class to be valuable to them. I want them to use their brains to grapple with the questions of (say) calculus to the fullest extent possible. If you're sitting in a room and showing up, why not do something while you're there?
I do unimportant things ("flick", as we used to say in college). I enjoy reading blogs about fashion, computer programming, fitness, recipes, home-schooling; I do online shopping and window shopping now and then; I read stupid books and listen to low-quality music (Katy Perry? heh -- more likely the video linked below these days). But I do try to enjoy all these things -- I do these things because they give me pleasure. I try to skip events that are not going to give me pleasure or serve some other goal of mine, and if they're serving another goal I try to keep that in mind while I'm there. The first important question of mathematics -- "Why do I care?" -- is one that I ask about other activities I take part in.
My students don't think the same way. They don't have the same urgency or passion. Probably because they're just out of high school, they think that it's normal and "adult" to do useless crap or sit around doing things that don't serve your goals for hours every day. Students who don't really want to do calculus show up anyway, day after day, even if they're failing, because they're "supposed to". Students who do need calculus blow it off even though it's going to be really useful to them next semester. I was no model of foresight; I did not appreciate how important Legendre symbols would be at some point in my life and thought they were kinda dumb. On the other hand, when I thought something was not important I just stopped showing up and did something more important for the moment. Why don't my students do this?
Instead, they show up but some refuse to engage. I say, "Think about this for a moment..." and they say things like, "No, I can't!" It takes incredible coaxing to get some people to think. They seem afraid that it will be physically painful, perhaps give them a rash or a seizure. I need to say, "Think about this for a moment -- I promise it won't hurt -- what do you have to lose? -- and you might figure out the answer." Then they might try it, and often an answer indeed does appear. Astounding! And even if an answer did not appear, they were not harmed by the experience.
Why just sit there sort of ignoring the class around you, wishing you could sleep, when you could actually be in bed? Why sit there staring out the window if you could actually be outside? And if you think it's so important to be there that you stay, why not do something related to the content at hand?