Not really a hard question--but the pat answer "I want to help veterans" doesn't have enough meat to it. Research the area for which you are interviewing (VHA, VBA, NCA etc) and note the awards and recognition that area has received in the press. See how that connects to your skills and interests, and mention it in the interview e.g., I noticed that NCA is recognized for their customer service. I pride myself on my customer service skills, and feel I could both contribute and learn from the NCA team.