Overgeneralizing can be subjective but more or less patterns emerge.

Nintendo fans care about that style of 1st party software that Sony or MS can’t capture. Their games are whimsical, for adults they make you feel like a kid again and recapture that feeling of wonder and fun you had in the younger days. For kids it’s likely the first console they have because their parents had Nintendo or just figure its the most family friendly brand. You don’t play Nintendo for the power and bells and whistles; you play for titles that are unique to gaming and ones you call your friends over to play on the couch.

Xbox and PS fans are simpler. Games that look lifelike over whimsy. We want our consoles and to have the best specs because that means better looking and performing games. Complex narratives with nuanced characters. Mature content (not blood per se but more like adult themes) and quite simply the best 3rd party content.

PS gamers tend to love 3rd person games. “Single player 3rd person story driven action adventure first party” games they can’t seem to get enough of since Sony’s best exclusives follow that formula. I’d say Xbox gamers gravitate more towards some combination of FPS, racing, and WRPGs.

More like the opposite, the gamer buy the console because of what is available on it, his taste is prior to that.

Xbox and PS are pretty similar because they're top selling games are almost the same while nintendo fans are more loyal towards Nintendo games.

From where does 2/3 of Switch owners owning PS or Xbox show they are Nintendo fans instead of the opposite?

I bought the Switch for 3rd parties titles and got my PS4 for 1st party titles (God Of War, Uncharted 4, Wipeout and Spider-man were the only games I played on it) so I guess I can't really know why people buy a console or another.

And I guess if you're japanese or french, you would mostly go for Nintendo, american for Microsoft and the rest of Europe Sony

I'm a Nintendo fan myself, and as has already been said by others, it's their whimsical and fun-focused approach to gaming that appeal to me so strongly, as it makes their games great for the kind of escapism and de-stressing that's my main reason for playing games in the first place.

That said, I am not Nintendo exclusive. I also love me a good M-rated action game, which Nintendo simply don't make. Some of my favourite games include the likes of Resident Evil 4, Bioshock 1 and Infinite, and Hellblade. I'm currently on my second playthrough of The Evil Within on Xbox 360.

As a person I describe myself as imaginative, empathetic, highly anxious, and socially awkward.

My experience of being part of the Nintendo fanbase has generally been very positive. I admit it does annoy me sometimes that a lot of the more hardcore fans will defend absolutely everything Nintendo make or do no matter how crappy, but for the most part its an upbeat and passionate group with shared love of Nintendo's unique design philosophy. Nowadays most seem to be adults who grew up with the NES/SNES/N64.

Switch primarily for 3rd party titles??? Wow, that’s the exception to the rule if I ever saw it. 

PC, PS and Xbox are basically the same thing these days with the top played games being multi-platform.
Nintendo is probably the stand out as its more arts and legacy. Gone are the days with Sega vs Nintendo when it was more about craft.

It all goes down to who buys the main console will gravitate towards the games on it, however between Sony and MS and PC, they all fight to sell you their platform because you only really need one of them as the game libraries are almost identical. Unlike Nintendo, they don't have to convince you to buy their console based on hardware specs and features.

Sales charts show MicroSony fans are virtually the same. Nintendo owners go almost exclusively for Nintendo titles but who is to say if Calls of Doody or Madden weren’t yearly available that they also wouldn’t sell very well? I doubt they’d still make a dent in Nintendo software. PC is way too broad to make any generalizations.

It’s an interesting difference.

I’ve never really ascribed to one console. I always end up buying them all. There are certainly ebbs and flows as to which console has my attention at the moment, but consoles typically go through strong and weak patches of time.