1. They are probably extremely selfish.
2. If they think they need an NDA in a conversation, then they probably don't know how to think well enough to have an idea worth hearing.
1) NDAs are pretty much unenforceable. If someone asks you to sign one, that means they are very new to the business, and probably have a lot of other mistakes to make.
2) It shows they don't trust you. My rule of thumb is: distrust is a disease that NDAs won't cure, and indicates much deeper problems. If you don't trust someone, just don't share with them.