「？」「」
"Why aren't you going?" "Because I don't want to."
Literal
"Why go-not [explanatory-question-の]?" "Go-want-not [reason-から] [copula-だ-emphasis-よ]"
A how-to-answer-why question set. The question uses どうして + の for a casual inquiring tone. The answer stacks 行きたくない ('don't want to go') + から ('because') + だ ('is') + よ ('emphasis') = 'it's because I don't want to, I tell you.' This からだ/からだよ ending is the standard way to make a 'because' clause stand as a complete answer.