that's why

that's why

Used after a statement to emphasize the reason or explanation just given, especially in response to a statement or question using the word "why." A: "Why can't I go to the party with my friends?" B: "Because you have a huge test on Monday that you need to study for, that's why!" A: "I don't understand why we need to learn this stuff." B: "Because it will be useful to you for the rest of your life, that's why."
See also: why
Farlex Dictionary of Idioms.

that's why

a tag on the end of a statement that is an answer to a question beginning with why. (Shows a little impatience.) Sue: Why do you always put your right shoe on first? Bob: Because, when I get ready to put on my shoes, I always pick up the right one first, that's why! Mary: Why do you eat that awful peppermint candy? Tom: Because I like it, that's why!
See also: why
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • operative
  • the operative word
  • and that's a fact
  • where on God's green earth
  • take (someone) at his/her word, to
  • NTIM
  • that's my story and I'm sticking to it
  • "I see," said the blind man
  • checks notes
  • I'm sorry to say
References in classic literature
"I say, that's why I told you--so you would notice it, you know--that the sun shines, and all that.
Pollyanna
"So that's why! That's why!" a voice whispered in Princess Mary's soul.
War and Peace
"That's why he went over so easily," remarked the Lion.
"That's why he nearly blunted my claws," said the Lion.
The Wonderful Wizard of Oz
That's why my inventive friend was so anxious to go on this expedition when he learned Beecher was to go.
Tom Swift In The Land Of Wonders
That's why, intending to be in Venice some weeks, possibly all summer, and having some literary work, some reading and writing to do, so that I must be quiet, and yet if possible a great deal in the open air-- that's why I have felt that a garden is really indispensable.
Aspern Papers
You know we're utterly unlike each other, different tastes and views and everything; but I know you're fond of me and understand me, and that's why I like you awfully.
not of love, but of sins...are brought all at once so near to a creature pure and innocent; it's loathsome, and that's why one can't help feeling oneself unworthy."
Anna Karenina
"That's why you were so much struck when I mentioned to Zossimov that Porfiry was inquiring for everyone who had pledges!" Razumihin put in with obvious intention.
That's why they instinctively dislike history, 'nothing but ugliness and stupidity in it,' and they explain it all as stupidity!
Crime And Punishment
"You may laugh if you want to, Miss Rose, I know my things are queer, and that's why I hide 'em; but I don't mind since you've found me out, and I ain't a bit ashamed except of being so backward at my age," said Phebe humbly, though her cheeks grew redder as she washed out some crooked capitals with a tear or two not yet dried upon the slate.
That's why they work so well, I suppose, and why people let you do what you like with them.
Eight Cousins
Roberts adopted him, and that's why I don't know his real name.
The Valley of the Moon
That's why she is anxious for you to come to the house.
The Idiot
And that's why, to share them-" I faltered myself with the horror of it.
Turn Of The Screw