That's why... / That's the reason... / Therefore...
One of the most essential result connectors. Introduces the consequence of what was just said. Very natural in both formal and informal speech.
I was tired, that's why I didn't go.
That's why I told you earlier.
We had no budget; that's why we cancelled the project.
I didn't study, that's why I failed.
I love to travel, that's why I save money.