A line in the grass
You and a friend share a football field together. You have your side, and they have theirs.
There isn't any formal separation between your side and theirs. There aren't any lines or markers to signify a separation.
So you draw a line to separate your sides. It's clear now.
But then one day, they step over the line, so you point to the ground and say, "Hey, you're stepping on my side". They shrug and walk away.
The next day they do it again. You say, "Hey dude, you're stepping on my side again. My grass is gonna get ruined". Again, they shrug and walk away.
They do it AGAIN the next day, and you get really pissed off. Obviously the line isn't doing much. So you build a 4-foot wall to keep them from stepping over again. The field looks a bit uglier now, but at least your grass is looking healthy.
The boundary isn't there for them - it's for you. It's there so that our grass stops getting stepped on.
Setting boundaries is a lot like sharing a football field with someone. You can't get mad at someone for stepping onto your side of the field if you haven't even drawn a line to begin with.