Be aware of how people around you are feeling, and how your actions could impact them. This sounds like kindergarten stuff, but being kind has always been about mastering the fundamentals.
For instance, I've been made to feel a bit self-conscious about my punctuality lately, but here's the reason this isn't one of those random rules of etiquette: boredom and scheduling. Assume you're meeting in a public place – not someone's home or somewhere they are preparing.
Imagine yourself being a bit early when the other person is a bit late. It's a bit boring, while you wait. Maybe you feel a bit of, am I in the right place? Maybe you're just milling around, getting looks from people because you're waiting in a place where Things Are Done, and waiting is weird.
Forget the clock completely. Because time is linear, someone has to be first and someone has to be second (and someone has to be last, if everyone is waiting until the group comes together). Understand that you get to choose whether you are bored, or your friend is. What type of friend would you be if you didn't assume that burden yourself?
Same goes for last minute scheduling or reschedules. You have myriad priorities, as do they. Is there any reason you should indicate they are a low priority? They have a limited schedule – is there any reason to not attempt to stake your claim as soon as possible? Treat other people's time like a valuable resource. Not something of infinite value, it isn't important to never reschedule. But understand that every time you reschedule them to do something else, you've told them, this other thing is more important than you.
And many things are! I pity someone whose calendar is full of only the essentials. But to communicate that to someone in your actions is not friendly.