WARNING: WALL OF TEXT
I don't know where to start. xD I owned gold, and what automatically made it my favorite was it was gold. I mean gold was awesome when I was like 6. It was cool, shiny, and expensive.
My first video game system was Game Boy. My first game was Pokemon Blue (though I prefer Red today) and I loved it. After I got the game, I started watching the TV Show, and it was awesome. The first episode, we see Bulbasaur, Charmander, Squirtle, and Pikachu, who were all in the first game as well. You also see a shiny bird flying through a rainbow. At that age you were wondering what it was.
After all the hype of the first generation, here comes a second game with over 100 new pokemon! It was so surprising that there would be new Pokemon, and it was awesome. The new starters are amazing and equally as B.A. as the first generation's. It was in color, and the sprites were actually legible. You play through the game with a new professor, a PkMnGear, and new wild pokemon, like an cool owl, a spider, etc. Then you catch the legendary pokemon. If you had Gold, you caught Ho-oh. If you were old enough to be very observant, you were thinking "This is that Pokemon from the first anime episode!" and you caught it.
Oh, on a side note, G/S/C had the first shiny Pokemon, and even a shiny Gyrados that you get the total opportunity to catch. A red Gyrados was crazy back then.
You beat the Pokemon league. Your adventure was over. Or so it seemed, but you get to go back to Kanto! You got to experience the first generation with a new story, better graphics, and cool new Pokemon. Brock had more Pokemon, Misty had a boyfriend, and Koga's daughter had the gym. Everything was new.
Now, I only got to the power plant before I lost the game on the beach, but I no-lifed Heart Gold and beat it completely, and it's just awesome.
With all these surprises and improvements to your favorite (?) video game, what wouldn't keep this game in your mind for the rest of your life?
That's why. :P
Edit: How could I forget. A new place with really high leveled wild Pokemon, Mt Silver, awaited you. You went through puzzles until you saw a familiar figure. A boy in a red outfit was there. You walked up to him and he greeted you with "............. ............."
If you had Yellow, you kept Pikachu, it's obvious. You probably caught Snorlax after getting it out of your way, and I'm pretty sure you collected all of the starters. You got Eevee from that guy, and if you had it in the second generation, you probably only played in the day and evolved Eevee... in the day.
I'm sure there's at least one person who kept that team, and when they played G/S/C they were essentially fighting themselves. You can't beat that.