I was asked that question. Man that was 11 years ago. I don't remember what I told them. 11 yrs later, I'm American citizen living the American dream.
Anyway Good luck to you.

Its very tricky on what they want on this question. Thank you for your response. Still waiting to hear more experiences.

I would focus my answer on these points (These are my personal opinions. You can 100% disagree with them though):

1. Family Tie: I have family here in Nepal so I would return to Nepal after study. Why would I live an alone life in the USA?
2. Entrepreneurship: I always have a dream to open my own business in Nepal. Or, I have my family's business which needs my support and help to grow.Thus, after graduation, I would definitely come to Nepal to achieve my dream.
3. Need of the nation: After earth quack, Nepal needs to develop rapidly to catch the pace of the world, and there are ample opportunities here in Nepal where an educated man power is needed. It will give me a chance to give something to the Nation where I am born and grown up.
4. Political Stability: Recent elections show that there is a high chance of political stability and stable government for at least 5 years. That helps many projects to commence. Thus, there will be a lot of opportunities for the people like me who would have the US degrees which is high in value in Nepal.

Like tito-satya said, strong family ties and entrepreneurship are very strong points for you to come back.
If you are only child or you parent , then it will be easy for you to explain your need here in Nepal for your parents.Also if you have kids or married, it will be easy and convincing.
If you plan to show your interest in business , it will be better to explain a detail on how you are planning to do so and how your education here helps.

If your parents have decent business , you can explain your interest on that and a plan to expand it when you come back from USA with the knowledge and experience you gain from the studies.

good luck