For me it ties back to the classic Golden Rule - "Do unto others as you would have them do unto you." It's a baseline level of behavior that I apply to anyone I meet regardless of what I know about them. That's why it's "common courtesy," because that level of empathy from me applies to everyone equally.

I would save a child about to be hit by a bus, but it's not because I respect the child. It's because if it were me or my child I would want others to do the same. I don't need to know anything about the child to have that level of empathy towards them.

Respect is different though. Respect is a response to something I know about a person. It's a difference in motivation in contrast to common courtesy. If I do something for someone out of respect it's because that person has done something that makes me want to go out of my for their sake specifically, not just because they exist as part of the general populace.

You actually hint at this a bit yourself with this line:

> out of respect for their existence

You aren't respecting the child in that instance, you're respecting the concept of existence. The child hasn't done anything to make you want to save them as an individual, but rather you have admiration for the concept of life and so you save the child out of respect for that concept rather than on behalf of the child as an individual.

And that's fine! Existence and life are incredible things that are indeed worthy of respect. But there is a difference between respecting those things and respecting the person. A person doesn't have my respect until they do something to show me that they, as an individual, are worth a higher level of consideration. Existing alone isn't enough for me to give the person respect because everyone exists. It doesn't make one person any more or less respectable than any other person.