My main is Garen, and I've played him since season 1. Took a little break in season 2 because of experimenting with Fratmogs Riven, but when that stopped being a thing I went back to primarily playing Garen.
Back when the original Guild Wars was still popular I played a warrior on there. That game had the typical MMO trinity of tank/heal/dps, but I wanted none of it. I made a tank that could self heal to the point where our guild's healer started bringing dps skills.
When League started to gain popularity a few guild mates and I tried it out, and Garen was one of the tutorial options at the time (if I remember correctly). He was a fighter, he was tanky, and he had the ability to heal up, so I tried him out. I got used to the playstyle and ended up enjoying him, even though I had to learn to deal damage with him.
Now I just know how to play him. I enjoy the playstyle and the simplicity, and while I do play other, more complex champions I always come back to Garen. There's just something about him I find fun, whether he is strong or weak.