Because without Nintendo I would have never been a gamer!

For me it's Nintendo's commitment to gameplay first.   Too many games nowadays focus too much on the cinematic  experience and not enough on gameplay.  Most people seem to like this, but I prefer Nintendo's commitment to simple fun gameplay over anything else.  

I do enjoy a deep story, but no matter how good a story is, poor gameplay will ruin it.    The reverse is not true (Any Mario game will show that). 

Plus I tend to play in short bursts and it can be months inbetween play of a particular game.  Unless the game is so addictive I can't stop playing, then it needs to be easy to pick up and play.  Nintendo titles with few exceptions always are.

In general, Nintendo makes a good selection of great game every gen, and almost always, without exception, an extremely well made game. Nintendo almost always makes a game which I think is "the best" game of that generation.

With the Wii, XenoBlade is a game of better quality then almost anything else I played this gen. Donkey Kong Country Wii, Metroid Prime 3 and Super Mario Galaxy also impress as well.

The DS to me doesn't have any really outstanding games, but a lot of excellent ones, most of which are better then the offerings on PSP (Pokemon, Advance Wars, Metroid Prime, etc).

Gamecube has Pikmin, Smash Bros, and Metroid Prime.

GBA has Fire Emblem, Golden Sun, and Metroid Fusion (didn't translate over to DS very well).

N64 has Zelda, Smash Bros, Mario Party, and Mario Kart

Gameboy has Pokemon, Zelda, and Metroid

SNES has Super Mario RPG, Donkey Kong Country, Mario Kart, and Metroid again.

I mean every gen, Nintendo makes or publishes one or two excellent games, and that's why I love them.

Sony is pretty good too, but I would aruge that they didn't produce a single outstanding game on the PSP, and the games they do publish on PS1-PS3 aren't as good as Nintendo's offerings. Microsoft isn't bad either, but they butchered some really great IP's and I don't recall Nintendo or Sony ever going that (Banjo Kazooie, Perfect Dark, etc).

However that said, Square/Squaresoft/Square-Enix is also an excellent company. Chrono Trigger, XenoGears, Fanal Fantasy's prior to X, and the ocassional title now and then also light up my radar.

Short Answer:

I love Nintendo for the JRPG's they colaborate on with Square, and some in house franchises. Games like Chrono Trigger, XenoBlade Chronicles, Golden Sun, Fire Emblem, Final Fantasy (1-6), Metroid, Super Smash Bros, console Mario Kart's, Paper Mario, Pikmin, Mario Party's (N64), some Zelda's (everything pre Gamecube/GBA + TP and SS), Pokemon, and Eternal Darkness, make me love Nintendo.

So basically their niche franchises or stand alone games make me want to buy a new Nintendo system, and their big games give me something to play between installments.

I've always been a huge Zelda fan and that was the series that has always been Nintendo's "system seller" for me. Same thing with Mario. I definitely have more memories with Mario; however, I've almost always perfered Zelda after I played my first Zelda game, the Minish Cap. Metroid, Pokemon, Mario Kart, Smash Bros., and Kirby are all other Nintendo exclusive franchises I enjoy. Technically the Super Nintendo "introduced" me to Nintendo, but I grew up with the 64 and GameCube. In all my years as a gamer (only 14 haha), I've only purchased one game console (the PS3) that wasn't made by Nintendo and no handhelds by any other company. So in short, I love Nintendo!

They make games where you put 200, 300, 400+ hours into.

Also, SUPER SMASH BROTHERS!

Bring this thread back to life! I want to hear more stories and reasons!

At 6 years old i recieved the SNES with DKC. Took long, like some months, to learn what videogames are and how to play them to finish only that first one. At the end i was really proud. So i looked and asked every time i could for some other games for that platform.

Long story short now i just love many Nintendo sagas, so i must buy Nintendo platform if i want to play.
I'm just a 24 years old gamer that love some N sagas and can't change no more his mind no matter.
(well only if they became ALL crap XD)

Well, aside from the Gamecube, Nintendo always has some exciting reasons to buy a new console. Other developers really just upgrade hardware.

I was introduced to Nintendo with the NES when the hot game was Super Mario Brothers.

Because despite them being really awkward and kind of retarded when it comes to pleasing the hardcore market with good hardware(Basically the Wii. 3DS is awesome, gamecube was awesome), they make the best games in gaming. They got the biggest heart or soul, they're games feel alive. Sony has terrible first party games, along with Microsoft, in my opinion, and I have had both systems(360/PS3), but sold them due to how terrible the games are(the hardware itself was better then the Wii, though).

Zelda and SNES Super Metroid