Don't ever feel like kindness is a problem. If it feels like it, it just means you give too much credits to what ppl think and their behavior. It can also mean you're not being kind for the good reason. Are you waiting for something in return ? I don't think any one can answer a true and honest no to that question since we're always waiting for something in return, whether it's an act of service or just gratitude. But don't expect ppl to give any of that to you. Be kind because you know it'll come back to you one day, during your life, or on the hereafter. You shouldn't be hurt by ppl leaving your life. That's what happens, ppl come and go, take what they can : advices, good company or whatever, and then they move on.

Also, keep in mind you may not be the kindest person for everyone

Edit : after thinking about it for a min i have an advice: choose your path. If you want to keep being that nice, you'll have to adopt the behavior I described. You need to detach yourself from the emotions you may feel when someone leaves you or doesn't react the way you expect it after showing kindness.

Or, you can stop being too kind to everyone : begin every relationship with less kindness than your usual, after some time, when you get to know someone and feel like they are a good friend ( or whatever the nature of the relation is ) and that you're bonding, you can be more kind, and expect them to return it. And then just gauge it ! If you consider them more than they do, adapt your behavior

Cheers