Hey OP, I'm a man and I probably have a couple of days a month where I'm feeling a bit flat or down, but if you asked my partner she'd probably reply that I have 1 or 2 a year.
As a rule boys aren't taught to be as emotionally available as girls, so we tend to bottle up this stuff, which may be why other posters are replying as they are. I'm much more likely to tell my mate down the pub that I've been feeling a bit crap than I am my partner. I'm meant to be the strong stoic one, that's how gender rules work, and no matter how much we're aware of the stupid patriarchal bullshit, it doesn't mean it doesn't still affect us subconciously.
Hormones are a factor too. Noone would be surprised if a woman said they had a couple of down days a month, because everyone knows there's a monthly cycle involved there too. What most people don't know is that men have a periodic hormonal cycle as well, we just lack the really obvious bit where we bleed out of our genitals for a week straight.
Only reason I'm aware of this is that as a teen I was having monthly blood tests. They kept coming back with a low white cell count which was concerning my Doctor. Went on holiday for a fornight so one month it ended up being 6 weeks between one set. Suddenly my white cell count was fine. Turns out men have a hormonal cycle which affects white cell count, and as a teen mine was particularly drastic. Stands to reason that this sort of thing is likely to affect mood as well.
As for reasons why I feel down, could be work stress, could be that something or someone has pissed me off, could be fear that I'm missing out on life ( a particular one during the pandemic), could be missing my Mum ( she died a couple of years ago). Some times though, its nothing. I'm literally just feeling a bit crap.
Last thing I'd want though is for my partner to start tiptoeing around me. Hopefully her cheerfulness (not that I want her to be artificially cheerful either ) will snap me out of it. On the other hand, if I want to wallow then I'll just take myself off elsewhere.