Do you know the reason?
No one knows the reason.
He is unpopular for some reason.
I wish I had a reason not to go.
I know the real reason for his absence.
I don't know the reason for her absence.
I know the reason, but I can't tell you.
There is no reason why he should resign.
I don't know the reason he is absent today.
I don't see any reason why I have to apologize.
For some reason, I'm wide awake and can't fall asleep.
To put it bluntly, the reason this team won't win is because you're holding them back.
For some reason, people have been avoiding me like the plague ever since I got back from India.
Just saying you don't like fish because of the bones is not really a good reason for not liking fish.