I'll explain how Garen's passive is SUPPOSED to be fair.
Most top lane champions have sustain built into their kit.
Garen has a low-decent amount of Health Regeneration (hp5) at level 1, 8 health per 5 seconds.
His hp5 growth is tied with the third lowest figure in the game, +0.5 hp5 per level.
The two lowest are Aatrox
+0.25 who has a passive effect on his E that heals him based on the physical damage that he deal, which encourages him to remain in combat.
and Mordekaiser
+0.3 who gets MASSIVE heals from his W
He is tied with

Of these, only
are really even loosely considered viable top laners.
All of the others CAN be played top lane all but of them are significantly better in other positions.
Of these 5, Riven is the only one that doesn't have any built in sustain or healing abilities, which is why it is so important for her to build lifesteal.
So if Garen is not able to trigger his passive, he has some of the weakest sustain in the game.
Playing against Garen, you want to keep him in combat as much as possible.
That does not mean you want to constantly trade with him! He will out trade you!
Most other top laners also have some form of ranged ability / harassment / poke.
Poke, get him to trigger his abilities, disengage while he runs at you, while his abilities are down, trade. He will try to disengage, let him. Then poke him again before his passive comes back up. .. repeat.
Eventually he will be too low to engage and he will need to leave the lane or you will kill or zone him off completely.