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 are additional reasons, but they pale in comparison with the reason.
If you want to retain the possibility of mentioning further reasons, try this:
One reason for it is that he is too lazy.
Then you could add:
Another reason for it is that he is still a rookie.