Which person does Jack refer to? What if you have 2 friends named Jack? Does "horse" refer to a member of a class of animal or something else? Sorry but your examples are full of indecipherable nonsense. But I guess if you just pretend that everything you write is well understood then there is no problem.
Capitalization slightly narrows a search space that is already narrow, since that is it's only functional use it should only be used when appropriate. If every rule was applied at every instance your writing would both become indecipherable and you'd subtly change your intended meaning. Better to be misunderstood by some than to water down your message and add class/prestige/formality/distance all of which are inappropriate in most writing.
I guess your teacher gave you that example, but you ABSOLUTELY FAILED to understand the meaning of their lesson.