Because of the DS/DS Lite, 3DS, DSi, DSi XL, and Wii. Also, IMA FIRIN MAH LAZOR!!!!!!

they make the best and most varied games out there
my first console was Snes, although i loved Sega too, Super Mario world and Donkey kong, alongside other exclusives like FF and DBZ super boutoden 2, made me fall in love with them, since then, theyve never disappointed me, so i stick with them..

Wow! You really went back there to find this. XD

"Expertly crafted video games that are able to stand the test of time"

Super Mario bros. 3 and Super Mario World you said?

Edit

hmm....even though i did have a NES, and had the mario games, the snes surely was the system that did it for me.
Super mario world, Yoshis island, the Donkey Kong games, i played those games numerously.

I really like my playstation products, but i've come to realize, that nintendo has a charm and trait in their dna as a brand that clicks with me.

They truly do look to innovate, and not just tweak something someone did and say they innovated,
they're humble,
they give credit where credit is due,
they hold to their brand promises and are not influenced by what everyone says they should do (their success is a tribute to why this is a good company value)
they dont bash the other companies,
and lastly, they make fantastic games and love making great games.

I came for Super Mario 64 and Ocarina of Time. I stayed for Metroid Prime (and will stay as long as Retro Studios is owned by Nintendo).

- A particular game?
Pokemon Ruby Version most definitely. That game was my life in my childhood, I've probably put more than 2-3000 hours into it (if not more) across 50+ files. Never got tired with it and still haven't (I'm eagerly awaiting the inevitable remakes :3)

- A number of franchises?
Pikmin, Mario, Pokemon, Chibi-Robo!, Animal Crossing.. I could list heaps more that I love, but these are the standouts, I've loved most. Pikmin 1/2 are enchanting, Mario is pure fun, Pokemon is indescribably incredible, Chibi-Robo! is one of the most interesting, creative and pure fun games that I've ever played and Animal Crossing has incredible charm that makes me love it, no matter how repetitive the Wii version was.