Achdus Yisroel
Achdus Yisroel (Jewish Unity) is a concept that expresses the unity of Jewish neshamos from their supernal source. This concept is the root of the parallel concept - Ahavas Yisroel. Chassidus explains in many places about the special unity that exists among all Jews, like one actual body, from their source - that they are all connected to the Ribbono Shel Olam, and every Yid is like a limb in the body, and without him - the body is not complete.
From the Perspective of torahEdit
The unity between Yidden is from the perspective of Elokus, meaning - since Hashem includes all Jews, therefore the unity between Jews is the greatest that can exist, and they are literally one, even though externally each Yid appears as a separate entity and seemingly is not connected to another Yid.
As a result, the love between Yidden is essential and cannot be changed, because every Yid is literally like another Yid from the perspective of his virtue, root and source.
A person's completeness is specifically expressed when all the limbs of his body are complete, and if one limb is missing - he has a blemish. Similarly, the completeness of the individual limb is specifically when it is connected to the complete body, and if it becomes disconnected chas v'shalom from the body - it loses its chayus.
So too with Bnei Yisroel - every Yid is a different limb in the complete body, and therefore the unity between Yidden is like the unity between the limbs of the body, that although externally it appears that each one is a limb unto itself - nevertheless it is part of the completeness of the entire body. And if chas v'shalom it becomes disconnected from the body - it is as if it never existed. Therefore, kol Yisroel areivim zeh lazeh, and as per the well-known explanation - that all Jews are mixed and intermingled with each other, because they belong to each other in an eternal connection from the perspective of their neshamos.
The reason that it's possible to have love of "v'ahavta l'reacha kamocha" is because of the concept of "Ahdus Yisroel", which means that all Jews are one entity, literally like one person.
Through Learning Torah and Fulfilling MitzvosEdit
Achdus Yisroel is expressed through learning Torah and fulfilling mitzvos, as this is the purpose and goal of Jewish neshamos coming down to this world - to fulfill the will of Hashem, and through this they express their unity, from the perspective of fulfilling Hashem's instructions which are equal for everyone without differences.