"You're like a chestnut," I told my partner.
"You're like a chestnut, you have these prickly burrs of rough edges and big boundary walls and 'how dare they think they can get close' and if they make it through the briar bramble it's a very thin skin and a succulent, good heart."
"Honey, are you eating human flesh again?" he glowered at me and we descended into a pile of giggles. But, as unoriginal a metaphor as it is (I know Jo in Little Women is called a prickly chestnut at least once, for many of the same reasons), some of our loveliest and most loved humans do have walls on walls of avoidant or anxious attachment, rigid boundaries, strict independence when interdependence would build intimacy, and a tendency to pretend they are hedgehogs, rather than human chestnuts. (You may recall a post where this author says some of these things about herself.)
This metaphor is pretty easily solved in nature - a chestnut ripens and the burr comes off, and then the skin is thinner and a more reasonable "boundary set" is in place to protect the nut itself. In real life it's a little more complicated, but we all work on it, relationship by relationship, hedgehog by hedgehog.
But not all failures in boundary-keeping or in where and how boundaries are in relationships are in placing too many. What about the many times where we set a boundary in words, but don't follow through on the consequences? Where there is no burr at all protecting us and we're getting knocked around? Or where we're trying so hard to let people in that we're acting like they're part of "us"? These are also boundary mis-steps we might make in interpersonal relationships.
The one where we let everything and everyone (all behaviors) through our boundaries is called having porous boundaries and while it can take many forms, I metaphorically like to think of it as building my boundary too big around myself, like a bounce house. I make too much "me" or "my problem" or "part of myself" for worrying purposes, including letting whole other people inside there to worry about - codependent relationships. (This is a slightly more extreme version of the metaphor than the chestnut - a bounce house does not exist in nature, and it solves itself only through exhausted collapse when there is no more air blowing into it.) At some point, we need to make the size of situation we're concerned with sustainable, or we collapse around the playing people inside. Encompassing LOTS doesn't make us "better at" boundaries - it makes us bigger at them, people pleasing, or less certain where we end and others begin, which is really what purpose our boundaries should be helping us determine.
Codependency and people-pleasing hurt the person doing the people pleasing because of the energy output, but also the person theoretically being pleased because they don't get to have an authentic relationship with you. It creates dependency in them and hurts their possibility of being a full and strong participant in relationship with you.
So, just as much as we try not to be closed off chestnuts locking our interiority away from our partners, we should try not to be bounce-houses taking all of theirs within ours.