Which is why/ That is why.

[Ashraful Haque]

A: I sent you a message. You didn't see it?
B: No. My new phone does't have a notification light which is why I didn't see your message/ that's why I didn't see your message.

I went through a few pages about this and learned there is a difference when it comes to how we write them. But I just want to know if we use them interchangeably in our daily conversations.

 

[Ashraful Haque]

B: No. My new phone does't have a notification light, which is why I didn't see your message
B: No. My new phone does't have a notification light. That's why I didn't see your message.


There is no difference in meaning.

This is exactly what I learned when I looked it up. But couldn't find if it makes much of a difference. Thank you for the answer.
Now I don't know if this the case but I feel like in conversations 'which is why' would sound smoother?

 

[GoesStation]

This is exactly what I learned when I looked it up. But couldn't find if it makes much of a difference. Thank you for the answer.
Now I don't know if this the case but I feel like in conversations 'which is why' would sound smoother?

They're equally "smooth". You might hear either one.

 

[tedmc]

"Which is why" probably sounds smoother because it continues as a sentence whereas "that is why" starts from a new sentence.

 

[Charlie Bernstein]

Right. A comma before which, a period before That.