"Setting boundaries is not a more sophisticated way of manipulation - although some people will say they are setting boundaries, when in fact they are attempting to manipulate. The difference between setting a boundary in a healthy way and manipulating is: when we set a boundary we let go of the outcome." (Robert Burney)
One of my favorite teachers used to describe space as being a variation of densities. There are no real edges to objects because molecules fly in to and out of objects at all times. These molecules do not just fly out a few inches or feet, but actually reach to the edges of the universe.
In order to move through the universe we need room or we will start exchanging molecules with other dense objects. And everywhere there are molecules (parts of our self, so to speak), so we can only bump into parts of ourselves.
Be that as it may, we do feel crowded when others are on top of us, either physically or mentally. Setting boundaries ranges from the octopus hiding in a cloud of ink to someone telling their mate that they'd like to have a few minutes alone in the morning when they wake up.
The expression "don't cross me" doesn't exactly refer to boundaries, but does infer that one desires to go down a specific path without any obstructions.
Some people fall apart when we get too close to them. They get angry and make us the culprit, even though they are the ones who create the discomfort (in their own minds). Others, when crowded, can simply move away a little, or ask another for a little more space.
What are boundaries made of? Are they lead, or are they "ether (believed at one time to be the common matter of space)?" How can we move from having lead boundaries (that no one can cross or see through, to transparent ether boundaries? If we are all congregated masses of the same thing then having bullet proof boundaries make less sense. We are open because we are everything. Everything embraces us because we are everything.
Denying this doesn't accomplish much more than the ostrich does by putting their head in the sand.