I typically don't sign an NDA when it's just someone's idea. If they've already invested a significant amount of their own money and look to have traction, I may. If it's a situation where I'm more like just a contractor coming in to an established company, I'll sign an NDA (and have signed short-term non-competes as well too if need be).
When I don't sign an NDA, often (this doesn't happen every day, but on the order of 6-8 times per year) I'll get some line about how this is a great idea and I'm a fool to let it pass. Or that the other person has to do this to protect themselves because I could just steal the idea and make all the money. Or some variation of one of those. I simply state that if they're looking for a true partner, starting off on by distrusting me that much, and making the relationship extremely one-sided (based on the language in the NDA as it may be), is a recipe for failure.
Recently someone approached me with an NDA, and had a business for something that millions of people need (not a want, mind you, a need), and they'd each be willing to pay $100/year for it. I wasn't told what it was until I signed the NDA, which I didn't. But I asked "how will you deal with the competition?" and was told "there won't be any". "Really? You're creating a market that doesn't exist right now, and generating hundreds of millions of dollars in revenue within a few years, and you think there will be no competition at all?" "Correct".
I politely declined at that point, and pointed out that even if I'd signed the NDA, I wouldn't be able to 'partner' with someone that naive about business.