Heidegger points to death as the only thing that Daseins have as their own. We are constantly being confronted with other beings in the world and are forced to evalute their appearance and so it only makes sense that we would do the same for ourselves. And when confronted with the mortality of others, it seems likely that the mortality of the self would become more apparent. This sort of thinking seems to suppose that my being-in-the-world depends on other beings being-in-the-world. If we were to imagine a world without other beings, with no point of ontic reference (and therefore no ontological reference), then how would we come to understand ourselves at all?
It doesn't seems to me that there is much of a difference between Dasein and Mitsein, between being-in-the-world and the being-with-others. At what time in my life am I not being-with-others? Even when I am alone I have knowledge of the existence of other beings. And beyond that, I am around things that I percieve to be artificial, with designers, manufactuers and possible users which all contain references to other beings (ontic sites that disclose the ontological). Dasein, this being that discloses Being, might just be another way of giving an unidentifable designation, like that of a soul or essence. And perhaps Dasein is not like this because of it's mysterious nature. But if I can not really know what Dasein is, then can it exist at all to me in a functional manner? Maybe the Dasein is just a personal will, the ability to take in all the other beings being-in-the-world and being-with-others (which I really do think is the same thing) and not subscribe to a "they" ethic that can truncate or stifle you as an individual. But it seems that my designation in society, in the being-with-others, is one that allows for very little personal say and is deeply entrenched in the theyness that makes up our world.
So if I am always in the presence of other beings, in both direct and indirect ways, and am constantly confronted by other beings being-in-the-world, it seems as if my greatest source of angst and anxiety would be my own being, because I cannot fully know it, yet am confronted with it constantly. And while my death is my own most possisbility, the end of my Being and the only experience I can never share, anxiety about my own death is actually just anxiety about my own Being because no one, not even me, will ever really know it.