Hi Praveen,
Thanks for A2A.

This is one of the typical problems everyone faces who prepares for a management interview. There are a couple of ways to tackle this:

Rethink from your previous experiences - You can rethink over if there is any reason other than a job. What is the sort of lifestyle you expect, what position are you looking at,  what do you dream to see yourself as in 10 years and link the answer to these things.

Co-curriculars - Every person would have done some or the other activities in graduation tenure. Some depict team Skills, some depict creativity, some leadership etc.  Link the answer to how each of these events drove interest in you.

Academic projects - if you have done any project where you worked in team, took lead or managed team members, you can portray that as the starting point of your realisation that you enjoy doing such things and mention that this spark have you the reason to explore more and go I to management field.
People skills - if you are an extrovert or travel a lot or like talking to people these could be the driving reasons or trigger points for looking at management as a career .

Thought leaders - You can take examples of how you read some books of management or heard some talks by management experts or industry leaders and how it drove you towards management as a career
.
Hope this helps

It has to be your answer entirely. Connect it with your real life examples and tell how as a manager you are competent to take on more responsibilities etc...