If you were to look in a thesaurus (online or otherwise) under nice, you would find the word kind. We use these words interchangeably on the regular, but I propose that they are not synonymous. In actuality, there is a wide gap between the two. The distinction matters because we often take kindness for granted and give niceness more credit than it deserves.
Being nice looks like polite gestures, compliments, and human courtesy. Being kind looks like servanthood, altruism, and generosity. Niceness is fine and dandy, but kindness is what would make the world a better place.
Let's go to the source, shall we? Niceness is rooted in the aim to please. It is externally motivated by the opinions of others. It is surface-level and self-sustaining. Rarely do we ever do something nice—holding the door, complimenting someone's shoes, obligatorily asking how the cashier's day is—without expecting at least a "thank you" in return. You sacrifice nothing in being nice. Half the time we use it as a tool to placate someone or to get on their good side. Describing someone as a "nice person" is what you say when you've only had one interaction with them in which they did not directly offend you. The criteria isn't exactly prestigious. Niceness can come across as saccharine, so why would you choose artificial sweetness when you could have the real thing?
Kindness, on the other hand, is rooted in love. Kindness comes from a higher power through the goodness of one's heart. Being kind entails actions like intentionally utilizing somebody's love language, paying for the person behind you in the drive thru, or going out of your way to make a stranger's day. Kindness knows no ulterior motives, but is best friends with compassion and empathy. Kindness causes you to want to invest in people's lives. Proverbs 16:24 says, "Kind words are like honey—sweet to the soul and healthy for the body." Some translations use "gracious" or "pleasant" instead of "kind," but not-a-one uses "nice." Parents tell their kids to "play nice," because children aren't necessarily developed enough as people to act on the deeper trait of kindness. The best people I know are the ones who ooze kindness day in and day out.
There is nothing wrong with being a nice person, don't get me wrong, but I encourage all of us to strive to be kind people. Compliment someone's disposition instead of their designer handbag. Hold someone when they are hurting instead of just holding the door.
A little bit of kindness goes a long way.