Why is Mr. Collins constantly complimenting everything he observes?

Really his constant compliments are really conceit. Mr. Collins' upbringing by an "illiterate and miserly father" along with his unexpected good fortune in finding a patroness like Lady Catherine has led to his lack of good sense and his strange combination of obsequiousness and self-conceit. Now that he is settled he wants to "make amends" for inheriting the Longbourn estate by marrying one of the young ladies in the Bennet household. He wants to appear a gentleman even though he knows that he inherits what he sees.