给你看些例子.
1.The Boss is quite down these days. He seems to have something weighing heavily on his mind .
老板这几天沉默寡言,看起来好像心事重重的.
2. The professor doesn't measure up to her expectations and she's quite down in the mouth about it.
这位李教授叫她‘失望’,她在那里愁闷.
所以“quite down ”可以理解为忧伤、伤心等一类词语.有点类似于安静的哀愁着.
Why you quite down?大致可以翻译成：你为什么这样伤心/忧伤?