Why on earth? sao … lại

You know sao and tại sao for asking why. Put lại before the verb, and the question shows surprise, like "how come?" or "why on earth?": Sao em lại khóc? Why are you crying? It says you didn't expect this and want to understand.

The order is sao, then the person, then lại, then the verb: Sao anh lại nói vậy? Why would you say that? (in Hà Nội, nói thế). Tại sao … lại works the same way and sounds a little more serious: Tại sao chị lại nghỉ việc? How come you quit your job?

Here lại doesn't mean "again". It marks something as unexpected, and sometimes as a complaint: Sao con lại đánh em? Why did you hit your little brother? So say it gently, with the person's word, when you mean concern rather than blame.

Two short ones are everywhere: Sao lại không? Why not? and Sao lại vậy? How come? (in Hà Nội, Sao lại thế?).

Answer as usual, with vì or tại vì (because), or with just the reason. A plain Sao em về sớm? simply asks why; lại adds the surprise.