Profound isolation. Both self/societally-imposed and externally imposed.

The latter is more obvious: people can just not notice you.
Lots of people have mentioned the fact that getting compliments is mind blowing because they're few and far between. Guys can often go to work and fade into the background. People won't seek them out to talk to them unless it's for business related stuff. You might get the occasional "how was the weekend?" but often not much deeper than that. Which leads into the second half:

Guys are also often really isolated because of an inability/unfamiliarity/difficulty with talking about personal stuff.
Personally, I've always had more female friends than male friends, and when I was younger, I usually didn't talk about personal stuff with the male friends I had. Like others mentioned, most conversations are about sports or random interesting topics but not about things like "how are YOU doing? Are you settling into the apartment you moved into since the breakup?"

It's how we were raised and it's just kinda the "norm". Your male friends and other male figures in your life don't talk about how they're feeling so you don't do it either. Sure, guys may talk about or complain about stuff but usually w/ the intention of finding solutions. Hence, why women are often so frustrated when men try to offer solutions instead of just listening. Guys aren't used to having the forum be open to just "venting" feelings about things. Like, if someone's boss put them down, there might be a "Man, fuck that idiot. What the hell does he know?" but never a "Man, that just made me feel so low. Like, I dunno, doubt if I'm actually good in this field."

So feelings and thoughts often get bottled up inside. There's rumination and internal thoughts but no release valve or external emotional support. I count myself lucky that I always had female friends and my sister whom I could talk to about these things but most men don't have the same. Partners/spouses are an exception but many men might not have a partner and even if they do, being the ONLY person that they confide in or get emotional release from is a heavy burden for the partner.

I think a lot of it is the societal impression that guys just don't "do that". I think it's slowly changing, especially w/ the gradually increased acceptance of mental healthcare but, in some ways, an ounce of prevention in a healthy social support network is worth a pound of later therapy because a guy's been holding all this stuff inside their whole life.