Zed looks up to Doc. Not just because he is an actual doctor of medicine as opposed to someone who only knows science from those kits for children, but because he is also an ovine hybrid. Sure, Zed is a sheep hybrid and Doc is a... actually, what is he?
Anyways, the first time they met, Zed challenged him to a headbutting contest. Up to this point, the only hermits he had headbutted were Tango and Impulse. Impulse had horns so Zed could be a little rougher with him. Tango didn't, so Zed had to be softer, often using it in a less playful, more affectionate way.
Zed largely underestimated Doc.
For starters, Doc is much larger than Zed. Second, he is made of metal.
Do you see where the story is going?
Sparring the boring details, Zed learned something that day but was too concussed to remember what it was.
Tango and Impulse are pretty good at keeping Zed from the concussion zone but It doesn't always work.