I think the intent behind question 3 is to find people who want to work on something that defines their life enough it makes it on their obituary.

Selecting for life passion, not temporary dreamers.

Bit obvious for that, isn't it? (Although I guess "willing to say whatever they think YC wants to hear" might be part of the filter too, possibly in multiple ways)

If you don't have anything interesting to say about yourself or your ambitions, maybe you just aren't that interesting of a person...

But maybe not?