The word kind has become very loaded in recent years.
It can be a bit of a catch-all, meaning considerate, thoughtful, loving, generous... but also self-sacrificing, putting-oneself-last.
Our society strongly expects all of these from women and girls. Interestingly, we only expect the first few from men and boys - and even those much less than girls. We never expect boys to put others ahead of themselves. Self-sacrifice is only expected of girls.
It's a very strongly gendered expectation in our society. And it's been weaponized against women. It's a powerful tool used to shame and bully women into dropping their healthy boundaries and allowing other people's demands to take precedence over their own reasonable needs.
I now cringe at the word 'kind' and would never use it to my DD. Instead I use more precise words like 'thoughtful', 'considerate', 'good friend', 'fair', 'generous' (generous being applied to emotions, not just things). And I'm clear with her that these behaviours should flow both ways in any relationship. She should expect to be a recipient as well as a giver.
Interestingly, a side-effect of teaching children to expect to receive fair behaviour from friends as well as give it is that they are much more attuned to seeing bullying behaviour from someone else towards a third child as unacceptable, and to stick up for the victim against the bully.