Nice an
adjective- 1.giving pleasure or satisfaction; pleasant or attractive."we had a very nice time"
Kind an
adjective- having or showing a friendly, generous, and considerate nature."she was a good, kind woman"
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I guess language plays an important part and the words we choose to associate with. If someone was wearing a pretty dress I might say oh that dress looks nice on you. I wouldn't say kind. If someone helped me pick the shopping up off the floor that had spilled out of a bag... I'd say, that's very kind of you to help. I wouldn't say that was nice of you to help." MW 2020Being kind is an act that may not be seen as being nice it may be offering someone a mint , it maybe acknowledging their need and meeting it with no expectations.
Which leads us to nice ... nice is a trickier one as it depends on social norms , like saying you look nice or being being polite when asked if something looks ok ... being nice can also on the flip side be an act of passive aggression if the person you are being nice to doesn’t appreciate it or they themselves are being nice .for example what do you want for tea ... and the reply is oh I don’t mind you chose . Leading to a battle of who is being seen as being the nicest.
The difference to me is a kind person acts from compassion a nice person acts from wanting to be seen as being nice ... A kind person will go against societal rules to be kind . A nice person won’t for fear of not being seen as nice.
Be kind.