I define "nice" as doing nice acts, but with an underlying motivation for something in return. Most people, and all "nice guys" do this. This is a form of manipulation. Ill do something nice for you, if you give me something. This is why nice guys are so often labelled creepy, and it is why nice guys always finish last. They're too afraid to stand up and directly communicate their wants and needs.

Whereas being "kind" is to do something nice, simply because its the nice thing to do. Kindness comes from an abundance mentality. Kind people are the people you know who are infectiously happy, that many people gravitate towards because they are overflowing with value. They are leaders and mentors. They do what is kind out of the good of their hearts, not for any other motivation. And, unlike nice guys, they aren't afraid to withhold their kindness when it isn't suitable to them, or the recipient doesn't deserve it.