The opening chapters of Heidegger’s Being and Time establishes the structural reality of existential being. Again, Heidegger is attempting several things in his great treatise, but the boiled down “to the point” project is that Heidegger is attempting to recover the philosophy of metaphysical ontology (being) and, by this recovery, avoid the problems of nihilism, relativism, and establish the foundation for universal truth again (related to the nature of being). So it would only be natural, pardon the pun, that Heidegger attempts to establish the foundation of being in the early parts of his work.
Having established that being exists, and exists in the world, therefore denying Buddhist and Gnostic anthropologies that assert true reality as the immaterial realm of spirit and that this life is illusory, Heidegger moves into a radical revision of classical anthropology. Apart from a few sects of ancient Greek philosophy, Greco-Roman-Christian anthropology maintained that man was a social and relational animal. Man exists in a world with others, thrives in this world with others, and his very nature and well-being is dependent upon forming relations with others. Being-in-the-World as “Being-With” is Heidegger’s revised account of man’s relationality.
Since man exists in the world, man, by default, automatically has a relationship with the objects of the world. The I, returning to the I-Not I distinction in German romantic philosophy, is always with the Other. Being-in-the-World presupposes two things. That I exist in the world (of objects) implying a connection with the world which establishes my “roots” in the world (bodenständigkeit), and that I exist in relationship with others. Here Heidegger challenges the ontological philosophy of reductionist materialism and atomism head on. I am not opposed to the world per se, i.e. seeking conquest of the world which prevents any sort of connection or attachment to the world (bodenständigkeit). Furthermore, I cannot exist as an isolated, solitary, cut-off (atomized) being from others; others exist and I must acknowledge this reality. To deny this reality is to deny reality itself.
As such, the mitsein (being-with) is a fundamental reality to the existential nature of being. Being is not merely individuated, isolated, or atomized (as Hobbes, Locke, Spinoza, and the classical liberals maintained). Rather, being exists with the intention of being-with-others; hence why, materially, I exist in the world (perhaps refer back to Genesis 2 where Adam exists in the garden, though “alone” without a human companion, he still exists with-others: the world, animals, etc.) and why, ontologically, I exist with others (I am not the only human in the world because existence requires the other). Being alone is not good, in the classical sense, because being alone is a privation of the fullness of being which entails being-with; St. Augustine argued in Confessions that evil (that which is not good) is a privation of nature (that which exists and that which exists is good), thus, to be alone (being-alone) as a privation of being-with is something that is to be avoided as it is a privation of man’s ontic nature.
There is much more to Heidegger and Being-With, but this is the most basic understanding of Heidegger’s ontological anthropology. Being-in-the-World automatically ensures Being-With. Minimally, we can say that Being-With includes a relationship to the world; worldness. I am not the world, however. Thus, the I-Not I distinction remains. As such, the world that exists and I am in is pluralistic.
Moreover, as I exist I cannot have come into being without the other (it takes two to reproduce). Thus, Being-in-the-World not only entails being-with-the-world, it also presupposes and includes being-with-others. However, the other is not I either. So while I exist with others this relational existence with others is equally pluralistic.
In establishing the structural reality of Being-in-the-World, Heidegger challenges, based on existential reality, the liberal claims of ontology: That man is equivalent (or reducible) to world (thus destroying the I-Not I or Human-World distinction), and that man is an atomized, isolated, and solitary creature. Heidegger engages in a sort of charming ironic criticism between the lines too. Facticity, he is arguing, cannot be atomistic or monistic. The facticity of existence should affirm the opposite of what the classical liberals arrived at. That is, facticity affirms plurality and the social/relational nature of man’s existential existence.
That said, the crisis of the present is that we have deceived ourselves into the liberal privation. As such, man is cut-off (uprooted) from the world. And man is cut-off from others. This privation of connectivity with the world and this privation of relationships with others has led man into his petty, sorry, and nihilistic mode of being which Heidegger is trying to counter and show us a path out of. But to do this he needs to challenge the reductionist and atomized account of the world.