To exist is to differ?

Reading Latour, I came across this quote from sociologist Gabriel Tarde on difference:

"To exist is to differ; difference, in one sense, is the substantial side of things, what they have most in common and what makes them most different. One has to start from this difference and to abstain from trying to explain it [...]" (Tarde (1985/1999) Monadologie et sociologie: 73)

There's a point here which I take: to exist is to differ. That's true, we are all different. For Luhmann and system theorists, difference is the source of existence, as something comes into being by drawing boundaries around itself and, in this process of boundary-drawing, it sets the difference between what the system is and is not. My favorite example: a nation (pick one) comes into being not by virtue of historical necessity, but by delimiting itself along particular lines such as ethnicity, religion, language, territory. In this process of defining who the nation is, it sets the boundary separating it from who the nation is not.

But that's also not that obvious: why do we see the difference? To what extent our individualistic culture is not a source of seeing the difference, obsessing with it? Why are we different first and foremost, instead of similar (for instance, because we have one heart, one pair of lungs, one mouth and so on)?

I guess this comes down to the old constructivist debate: why do we see the tree as an individual unit and we do not see 'tree' as a whole, including the soil in which its roots are, the air which the leaves breathe etc. In other words, what is the principle of classification which informs our vision? From this perspective, I have to side with Foucault and Skeggs (cited in last post): the principle of classification is always partial, always connected to a particular distribution of (access to) resources in society (sorry Latour, I guess I am not an ANT scholar). And I wonder to what extent our principle of classification is not difference as a source of mistrust and different moralities?