How long as it been?
At some point in your life did Nintendo, the company of most of your childhood, let you down? When was it? Did it happen during the days of the Snes? The Sega Genesis or 3DO turned you against Nintendo somehow. Were there just too many games that were the same to you on the Snes. How about the Virtual Boy? That was a nasty little handheld/head held device. Did the 14 games released in America piss you off? There are some who thought it might have been the Nintendo 64 that ended their love affair with Nintendo. The system had too many dry spells when it came to games. The graphics were foggy and choppy, and there were times when the framerate suffered.
Then came the GC! Everything was supposed to be different. Nintendo's Freshmen year of 3D was over and they were back and better than ever. No DVD, great but not great graphics, No Mario at launch, more game droughts, and what was the deal with that controller? Maybe even that was not it. The Wii came out and you were intrigued. What is this motion controller that everyone is talking about? Then you got Wii sports, and it was a lot of fun. Then another party game came out, and another and another, and by then you were not having that much fun anymore. In its later years the big game releases got farther and farther apart, 3rd parties still did not understand it so no big ports or multiplate games were coming out anymore other than COD games. You were tricked with rail shooters and games that were missing content that the HD brothers had.
Is any of that it? If not that then maybe you just grew out of Nintendo. Maybe you are older and just want that hardcore FPS, RPG, Adventure game, etc. that other systems may have to offer.