I hear you, and it's a good point. The counter argument starts with a question - do you give your best every day you go to work? If you do that's great, but most people don't. And while being a professional hockey player is a very special and unique and well-paid job, it's still a job. And these things are always relative - most of these guys have never known anything else. So human nature dictates they will have good days and bad days, better efforts and worse, even in a job that many of us fans consider more a dream hobby than work.
The modern player is told often how lucky they are, how privileged, how short careers can be, how much they should cherish what they do while it lasts, and I'm sure most of them genuinely buy into all that. But in the day-in, day-out of the grind (and stress), inevitably that message gets lost or obscured sometimes. The length of the season tells against it too - if any player, no matter how fit or talented, gives 100% through 80+ games, they'll be burnt out by the time the crunch stuff even starts. So right from the beginning they're having to play the long game, and that's a slippery slope.
The challenge for the coaches and staff is to keep the players up and about, motivated and engaged, as much as possible, while monitoring their minutes, energy, fitness etc. at the same time as the months tick by. Last year's Bruins had the motivation in spades, but I'm not sure their energy and fitness management were optimal. Let's see if they can do better with that on the run home here.