To find the purpose to explain or justify an action, decision, or conviction, use ‘for’, as:
There are several reasons for traffic jams.
The teacher inquired about her reason for leaving the school.
The relationship between love and reasons for action is highly interesting and complicated.
To find the basis or motive for an action, decision, or conviction use ‘of’, as:
What are the reasons of traffic jam?
A joint Russian-Polish commission has been set up to find the true reasons of the recent plane crash.
What are reasons of coughing?