Why did Ginger hate the harness?

Over time, Black Beauty came to hate the harness because his owner regularly demanded the check reins be tightened further. Tightening the reins pulls the horses' heads to a more erect position..... and it hurts.

I had been driven with a check-rein by the dealer, and I hated it worse than anything else; but in this place we were reined far tighter, the coachman and his master thinking we looked more stylish so. We were often driven about in the park and other fashionable places. You who never had a check-rein on don’t know what it is, but I can tell you it is dreadful.