。
How can you be so optimistic about the future?
Literal
Future's matters [object-を] how that-much can-be-optimistic [explanatory-んですか]?
どうしてそんなに ('how, so much') is a compact set construction for 'how come you're so X.' 楽観できる is the potential form of 楽観する ('to be optimistic') — 'can be optimistic.' んですか is the explanatory question form: it asks not just for a yes/no answer but for an explanation or reason behind the observed fact.