Self-awareness leads to being nice because the more comfortable you are with yourself the less you'll get your stuff on other people. It's really easy to get caught up with the things going on inside your head and acting out on others in an unconscious manner. Maybe you've seen a boss fly off the handle at someone or a significant other shout at her partner. These are examples of actions that aren't self-aware because people are doing them automatically rather than with careful forethought.
Self-awareness means that you've taken the time to know yourself so well that you don't do things unconsciously. You're happy, balanced and able to treat yourself and others kindly. There's a myth in our society that being nice is being weak when it's actually a positive sign that someone is healthy and balanced. Please keep in mind that I'm not talking about the syrupy-sweet niceness of someone who really isn't happy or is trying to manipulate others, I'm referring to the kind of niceness that comes from someone who feels genuinely and deeply great about themselves. What will you do to be authentically nice?