Gen 3 (about 2003 or 2004) as a 4 year old. It was anime and cards until I got my first game in 2005, MD: Red Rescue Team. I remember seeing Ruby, Sapphire and Emerald but nothing looked cooler than RRT at the time. I still would have liked to get a main series game during Gen 3, but I don't regret Mystery Dungeon one bit. I collected many cards up until the end of the Platinum series. I would eventually go back in during BW, but leave again. (Went out with a bang, Shiny (alternate colour) Emboar.
My first main series game was Platinum, after my little brother got Pearl the year before. I loved that game so much and there was so much I didn't know. I was a really bad trainer through most of my games (probabably everything before X, where I wisened up), using my masterball on the mascot and having nothing but the most powerful moves of the same type on one Pokemon. I loved the Hoenn and Sinnoh anime, then dropped off after the start of BW. I loved all the movies as well and wanted to have all the legendaries for each title.
Pokemon will always have a special place in my heart. It's not too expensive for me, only buying the year's game, sometimes having to buy a new system (2DS when XY hit) and provides a ton for what you spend. I'd say my love for it is because of my grandma, who brought lots of cards and my first gaming system, the Gameboy Micro and Mystery Dungeon later on.