The tendency to say "yes" is often the byproduct of certain cultures and one's upbringing. This tendency is hard to break.

On a side note, I have always appreciated how candid and honest Europeans are. They speak their mind, they don't sugar coat anything and they are very matter of fact. On a tech project, this sped things up A LOT.

Similar thoughts from Derek Sivers (founder of CD Baby): http://sivers.org/hellyeah (2009)