You had recently taken a visit to the Kanto region (for any purpose, you choose) and you had taken a stroll through Saffron city. When you take the stroll, you'd come to find a Fighting Dojo in the city, so you decide to have a try. After beating a few black belts and whatnot, you stumble across a man at the front, with two Pokéballs. "I see you've beaten every black belt here. So I'll let you choose between these two Pokémon." The man speaks, and you see a Hitmonlee, and a Hitmonchan emerge from the balls, and in your mind, you decide to pick Hitmonlee.

You eventually head home, and as you arrive you decide to let your new Hitmonlee out, and as it emerges from it's ball, it seems pretty happy that you picked it.