This question is asked to check how confident an employee is about themselves. Well, why would you come to the interview in the first place if you had even one reason for not being hired? Of course, you don't say this to HR, but that's quite understandable.
The Best Answer
I certainly can't think of any reason why you should not hire me, but I can tell you ten reasons why I am the best candidate for this job, and you should hire me. I am listing out the bullets here (you can download the attachment for detailed interview preparation along with this answer):
1. You believe in the company.
2. You are interested in the role.
3. You are qualified and technically equipped.
4. You can use this experience for this role.
5. You are ready to manage bigger responsibilities.
6. You have developed special skills/contacts/relationships over a period of time.
7. You value comprehensive growth and not just money.
8. You know the competitors, industry, and economic scenario.
9. You are a stable and growth-oriented person.
10. You have a strong grasp of technology and always stay updated.
I hope this helps.
Good luck,
Regards, Vaibhav