Making a "code" is like building a habit. Maintaining the code might be tiring at first, but consistently caring about others, just like consistently brushing your teeth, will inevitably get easier. If you truly are feeling exhausted/insane after treating people well for a day, its unlikely you think people are worth treating well to begin with.

Exactly. You weren't born with the instict to say "fine, how are you you" or "please" and "thank you." You were taught to do that by your parents.

Habits like this are a muscle that can be developed. Relatedly, the concept of ego depletion: http://en.wikipedia.org/wiki/Ego_depletion

I think most humans are kind by nature, since we're social beings. In a setting like a tribe or a small community, it's relatively easy since we depend and know everyone else. It's when we have to be like that to everyone, including strangers.. that's tough.