If you've ever been around someone who behaves unkindly you also know that their lives don't tend to be very happy. Even if they're wildly successful financially or have a lot of stuff, they can't enjoy it as much because they've been mean to others. No matter how hard they try to cover it up, behaving meanly takes a toll on them.
Being nice is important because it creates balance and well-being in your life. When you're nice you don't have to worry about the repercussions that come from hurting people. You don't have to make room in your brain for all the negative energy that is created when you're mean to someone.
Being nice makes you happy because, no matter what anyone else does, you can be secure in the knowledge that you behaved positively. Being nice simply means that you consciously decide to behave kindly. You'll wake up each day looking forward to building happiness in the world for yourself and others.