Reason can be followed by why… or that…
- The reason why I came here was to be with my family.
- The main reason why he lost his job was that he was drank.
- Do you know the reason that they’re closing the factory?
In an informal style, why/that is often left out.
- The reason she doesn’t like me is that I make her nervous.
- The reason I’m asking is that I’m short of money.
The normal preposition after reason is for.
- What’s the real reason for your depression? (NOT …
reason of your depression?)
Some people consider it incorrect to use a because-clause as complement after reason (as in Sorry I’m late – the reason is because I overslept.).