Early stage investing is largely about investing in the people, and their ability to execute, and less about the idea, assuming a reasonable business plan. VCs want to determine how steadfast and committed you will be to the company when the going gets tough. VC's ask this question if they cannot infer this from your background, employment history, or accomplishments. For early 20-somethings, there usually isn't a lot of history there to tell them, so they have to ask probing questions.

Once your time spent at a single company with high stress, multiple people, and fast growth is measured in years and not months, VCs stop asking you this question.