I'm a graduate student teaching a very difficult sophomore-level required major class, and this is to the professors out there who can't understand why some of their students, especially sophomores and juniors, aren't willing to dedicate their entire lives to succeeding in their class. It's because, for the first time, they *have* lives they're able to dedicate.
And that's very difficult to deal with. They need to cook for themselves, find a way to earn money, negotiate roommates, friends and relationships. A lot of them will be dealing with insecurity, loneliness, and mild depression. They'll make one mistake in a class, not turn in a major assignment or do worse on a test than they were expecting, and they'll get scared... especially if they were good students in high school.
Sure some of them will take that as a call to pull themselves together, but others will just feel too ashamed to think about it, and they'll run away. And at the same time they're taking your class, they're learning how to deal with failure and overcome it, how to deal with friends who are bad influences, how to survive rejection and loss, how to help the people around them. While I'm sure they've skipped class because they wanted to sleep in... they've probably also skipped class because they were helping a sick or distraught friend, they were working to catch up in a *different* class, they were too intimidated to come, or were too depressed to get out of bed.
So many professors and grad students were the quiet, anti-social, perfect students as undergrads. In fact, I've met a few grad students who I'm pretty sure skipped the whole 'social maturation' part of the process entirely. They don't understand. Life should never just be school. School is important, but life should be too. Any professor who gets righteously indignant when his students skip on the first nice day of spring deserves the sort of life he's living.
My class is difficult, and I do not grade leniently. It used to be a weed-out class before I took it, and the only serious change I've made is that I'm sympathetic to my students. If they fall behind, I suggest to them to drop by office hours, and then I get them caught up. I explain what they need to do to stay caught up, and why I think they, personally, are capable of it. I never guilt trip them. And if when they come it's clear they're very, very lost... I just smile and start from the beginning, with no weary sigh or chastisement.
In return, the students are honest with me, and they don't argue or make excuses, they do their homework and come to office hours whenever they get lost. And as a result, of course, they do well in the class. It's not so much of a weed-out course anymore. They tell me I'm a great teacher... but honestly, the only special thing I do is have busy office hours. And I try to make sure that my class isn't yet another overwhelming thing in their already difficult lives.