why not後加什麼?

If you love each other, why not get married?

要是你們彼此相愛，幹嗎不結婚呢？

Why not have the dish of the day?

為什麼不吃今天的精選菜？

Why not write to her?

為什麼不給她寫信呢？

Why not tell her you're quitting and have done with it?

為什麼不告訴她你打算辭職，儘快把這件事了結了呢？

I didn't want a part in the play, then I thought ─ stuff it ─ why not?

我本來沒想在劇中扮演角色，然後又一想——管它呢——幹嗎不演？