Grass and steel being my favorite types have polarizing reasons. Immediately getting into pokemon I favored a lot of steel types because of their designs. You got a pokemon like Aron that starts off as an adorable, little armored creature that evolves into a monster with armor ready for battle. There's also Metagross which reminded me of Metal Gear, and the giant X on its face just looked so cool to me. Then there's Scizor. A red bug-mon that's made of pure steel and has pincers for hands, as well as an intimidating gaze. (It is also responsible for becoming a bug type fanatic, although I much prefer grass and steel). I think what came down to it is all the steel mons looking very intimidating, yet simplistic. Like Skarmory just being a giant bird but made of metal, Magnezone looking like a futuristic UFO with 2 magnets for hands, or Bronzong being a giant floating bell. It just works.
As for grass, this took me a long time to be my favorite. I used to really despise grass-mons because I never thought they were powerful enough. They have many weaknesses, like fire, ice, flying, poison, and bug. Because of that reason I did not choose to use them, ever. That was until I got pokemon leafgreen and decided to use Bulbasaur because Venusaur was on the cover. That choice alone changed my entire look on grass types forever (even after losing constantly to Agatha). I began using status moves, leech seed, and sunny day more often. That helped me develop more strategies when playing pokemon, and it helped me improve as a player. I started giving other grass types a try, like Vileplume, Breloom, Roserade, and Torterra. By the time gen 5 rolled around it added some grass-mons that had designs I really liked. Like Serperior, Lilligant, Virizon, and Ferrothorn. With the release of Black 2, Grass soon became my primary starter choice for all regions (except Johto and Hoenn). Even afterwards I reevaluated many grass-mons I overlooked like Exeggutor, Sceptile, Shiftry, Cactrune, and Victreebel. After several years I can say I really love grass types because they taught me how to use strategy, like sleep + leech seed and sunny day + chlorophyll.
