personal desires are often projected out to society as a whole -- "well, everyone wants A," or "now that you have a good job you'll obviously need B." And as pointd out, while the truth universally acknowledged may apply to some (anxious mothers, for example), it doesn't necessarily apply to all (although Mr Bennet may partly have been trying Mrs B's poor nerves by his denial). It certainly sets the tone for the rest of the book -- a hook that pulled me in and made me want to read more.