There are genetic and other factors involved too. Genetically I've been stuck with a damn lot of hair to start with. I have a thin crown area on my head and a mix of coarse/thick (everywhere) and fragile/thin hairs (mixed in and around my crown) that I suspect are supposed to be all of the coarse and thick variety (much though I'd personally prefer thinner strands). The back third of my head has the thickest hair growth. In my particular case, I suspect that the low iron I'm currently on supplements for is causing the thinning and causes hair to fall out (mostly in the back of my head right now), while hormone imbalance from hypothyroidism has caused the front of my head not to grow as much hair as the back (aka "male" pattern baldness). The one good thing about having so much hair to start with is that it makes these things easier to hide.