Hi,
First of all, let me explain, why MBB and other consulting firms are interested in your motivation:
- Consulting is a long term game with long hours, high pressure and to cope with that you really need strong motivation specifically for this role
- Consulting firms also check your understanding of the role via this question
Characteristics of a bad answer:
- Non-specific cliche reasons (e.g. ability to work with multiple industries, great people, etc)
- Unstructured delivery (you tell it like a story to a friend in a very unstructured way)
Characteristics of a good answer:
- Structured (start with naming 3 reasons and only than drilling down in each of them)
- Specific (as many details as possible)
- Genuine (only real personal reasons without BS)
- Remarkable (interviewer should remember your answer after he leaves the room)
- Value originated (you need to demonstrate your strength in your answer and tie them to consulting industry)
Good luck!