I agree completely that there's an asymmetry to how society views obsessive behaviors, that spending countless hours running or working out or just plain working is virtuous while all kinds of inherently harmless habits get demonized. As you assert, once one stops pathologizing what we do, it's easy to see it as a healthy habit instead. Call it addiction if you must, but understand that it's a loaded word that connotes such harmful habits as using heroin. If you can't find a way to fit it into your schedule and neglect important other things, then I suppose professional help would be warranted, but that would be true of obsessive running as well.
Funny that you'd mention left-handedness. I've long believed that what goes on inside us is akin to that natural human variation. Why does any given individual simply start using the left hand (and probably kicking and batting lefty as well if into sports) when most of us naturally use the right? And weren't Catholic-school nuns notorious for brutally trying to "convert" lefties onto the path of right-handed righteousness, as if being a southpaw was somehow sinful or against a divine plan?
So then, why, if we set aside the highly artificial moralization of everything remotely sexual in nature, shouldn't it be equally natural for some humans with XY chromosomes and an "M" on the birth certificate innately feel that they should be XX with an "F"? The brain is an incredibly complex organism, and we are nowhere near understanding all of its mysteries and potential harmless anomalies.