I'm no psychologist but I am feeling reflective this evening so yeah... :)
I think often, a big part of feeling the need to be "nice" to everyone all the time by being super flexible with our boundaries is rooted in fear & not true, authentic kindness +

You navigate through life this way because you're subconsciously concerned that exercising your boundaries (for yourself & for others) means people won't like you or won't be kind to you or will ignore you or will leave...whatever the case

Or maybe even sometimes it's a defense mechanism. A voice that goes "Maybe if im really really lax & cool with them first and show how "good" I am, it'll dispel any possibility of them doing me wrong".

But things don't really work this way & the better way to self preserve is in doing exactly what it is you're scared of/unaccustomed to Set that boundary.
You take care of yourself better doing this & create more room to be kind to those around you in a healthier, genuine way