I believe that there is absolutely a difference between the two. An easy way to understand what it means to be kind is to think of the kindest person you know and think about what they do to show that kindness. The most important word in the previous sentence was “do”. I believe that being kind requires action while being nice doesn’t necessarily require any action. Here are a couple of examples:
A nice person doesn’t bully anyone, but a kind person would defend those getting bullied.
A nice person doesn’t make fun of those sitting alone, but a kind person invites them to join their table.
A nice person doesn’t speak disparagingly of others while playing sports, but a kind person praises their opponent on a good shot.
A kind person occasionally buys two milk shakes instead of one, so they can give one to a friend.
The golden rule sums it up well: “Do unto others as you would have done unto you.” In my teenage years, I had misinterpreted that rule. I had understood it as “Don’t do unto others as you wouldn’t have done unto you.” I would think “Okay, I wouldn’t want someone to hit me, so I won’t hit them.” That’s not what the rule says though. It is a principle of action and not inaction. If I would love for someone to serve me in a specific way, I should go and do it for them.
