They are also getting paid millions of dollars for playing a game for living.
So? Who’s making you pay their whole salary? Or anything if you don’t want. That really has little or no significance to the point of the OP
(Edit: also ignores/dismisses how much real, tangible dedication and sacrifice goes into “playing a game for a living” as if they’re little kids getting dropped off to practice for an hour twice a week, and a couple hours game on the weekend)
They’re human beings and money doesn’t mean they feel less emotion, are immune to many of the personal and mental problems the rest of us have;
not to mention they feel even more pressure due to that money not less, most won’t make enough to live the rest of their life off unless they majorly budget, and even still they likely feel pressure to maximize potential earnings; someone who is further down the line up, bottom six for sure, and even second liners to an extent are replaceable and feel more pressure to show up and show less weakness “for the good of the team” even to their detriment
then non financial reasons they feel emotional effects; they’re genuinely competitive and are probably of the mentality they might beat themselves up for messing up in any way; they’re loyal to their teammates and may feel like they’ve let everyone down etc. there’s been more and more acknowledgment across major sports the last few years of how some guys, even solid players having panic attacks before games
Everything is relative: if you even made the nhl for 5 years most of us would consider them successful hockey players. If you’re of them mentality required to be intensely competitive and get that far, you might feel like a failure because you fee you could have done more, but you never made it.
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I assumed the part of the OP you quoted wasn’t just about any and every example of fan criticism no matter how small,
but rather how noticeably many fans take it pretty far and even make it personal. Many players already feeling pressure are probably tempted to read what people are saying about them and you always notice the negatives more
it feels like some fan bases will go from loving their team entirely for a two game win stretch, to completely dumping on whichever player they perceived to have the worst game for the next two losses, shitting on them personally and or blaming them almost entirely for their teams loss, and or wishing they’d be traded.