> **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. 

You could even mention several reasons but then say "..but **the** reason for it is.." (with [emphasis][1] on **the**) to mark this reason as the most important and others, as negligible.

I will provide an example. Say, you want to retain the possibility of mentioning further reasons. 

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. 

or

> There's **a** reason for it: 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, he uses **the** and it's the key reason, from his standpoint. He thinks that it's the **only** reason, and *there are no other reasons at all.* 

Despite first mentioning other reasons, he uses **the** to display his opinion towards these reasons. 

So it's tricky. (0:


  [1]: http://slovari.yandex.ru/%D1%83%D0%B4%D0%B0%D1%80%D0%B5%D0%BD%D0%B8%D0%B5/ru-en