I can't say why people lie; they just do. Everyone has their own reasons for not telling the truth.
Eric CarrClifford Stern: A strange man... defecated on my sister. Wendy Stern: [pause] ... why?
Crimes and MisdemeanorsProfessor Snape: Why you insolent, little... Professor Lupin: ...Professor.
Harry Potter and the Prisoner of Azkaban[Mike and Sully have transported to Hawaii] Mike: Why couldn't we have been banished here?
Monsters, Inc.