@mark noble,
When speaking in general ontological terms, the word "existence" can be applied to all aspects of "being", but Heidegger for one delimited the term to mean the specifically
human process of contemplation of one's own being. In that respect I do not think "optimism" was a key issue, rather it was "authenticity" or "self-determination". Presumably, however, if one's own POV gives negative results, the person is likely to be drawn into externally prescribed modes of being like "religion", which H might have categorized as "inauthentic", thereby detracting from "existence".