> **A** reason for it is that he is too lazy.

This sentence looks unnatural, because the whole structure seems to imply that there's only one reason for something marked by the word **it**. 

> **The** reason for it is that he is too lazy. 

This indeed implies that there's only one reason. Or, that there could be additional reasons, but they pale in comparison with **the** reason. 

If you want to retain the possibility of mentioning further reasons, try this (imagine a teacher speaking about his pupil who failed to do his homework):

> **One** reason for it is that he was busy helping a neighbor. 

Then he adds:

> **Another** reason for it is that his grandma got ill, and he was visiting her.

Then he finishes his speech:

> **The** reason for it is that he is too lazy! 

Here, we use **the** and it's the key reason, from the standpoint of the person saying this. As the speaker may think, the **only** reason, and *there are no other reasons at all.*