Because your enthusiasm is your most valuable asset. Enthused employees tend to be a lot more productive, and also drive up the morale among their peers.
Sorry, I should have said: besides your competence, your enthusiasm is your most valuable asset.
You have had your entire interview to demonstrate your competence. At this stage of the interview, if you have not yet demonstrated competence, no answer to this question is going to get you hired.
Wow, no, that's not even remotely true. My years of experience and track record of delivering completed products is my most valuable asset.
Enthusiasm doesn't get the job done. Enthusiasm and passion are fleeting.
Honestly, I cannot express how horrified I am by your reply.