Reason can be followed by why… or that…
The main reason why he lost his job was that he was dishonest. OR The main reason that he lost his job was that he was dishonest.
Do you know the reason why she hates me? OR Do you know the reason that she hates me?
The reason why I came here was to spend time with my little daughter. OR The reason that I came here was to spend time with my little daughter.
In an informal style, why and that can be dropped.
The main reason (that) they decided to split was that he had an affair with a colleague.
The reason (why) she hates me is that I’m more successful than her.
The normal preposition after reason is for.
What’s the reason for your unhappiness? (NOT What’s the reason of your unhappiness?)
We do not normally use a because-clause as a complement after reason. It is possible, but some people consider it incorrect.
Sorry I couldn’t come – the reason is because I was ill. (This is considered incorrect.)
Expressions with reason
Do anything in reason = do anything that is moderate or sensible
Reason with somebody = try to convince him
You cannot reason with her. (= You cannot convince her.)
Reason somebody into / out of doing something
They reasoned her into accepting the job. (= They persuaded her to accept the job.)