The preposition for is used both before and after reason.
-What's the real reason for your depression? (NOT ... reason of your
depression?)
-I need to talk to you for two reasons.
Reason can be followed by a clause beginning why ... or that ....
-The reason why I came here was to be with my family.
-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.
Some people consider it incorrect to use a because-clause as a complement after reason (as in Sorry I'm late - the reason is because I ouerslept.)