From a larger tutorial on Common Lisp Programming Style, comes a nice list written by Peter Norvig & Kent Pitman surveying "where your 'Style Rules' come from":

• Religion, Good vs. Evil "This way is better."
• Philosophy "This is consistent with other things."
• Robustness, Liability, Safety, Ethics "I'll put in redundant checks to avoid something horrible."
• Legality "Our lawyers say do it this way."
• Personality, Opinion "I like it this way."
• Compatibility "Another tool expects this way."
• Portability "Other compilers prefer this way."
• Cooperation, Convention "It has to be done some uniform way, so we agreed on this one."
• Habit, Tradition "We've always done it this way."
• Ability "My programmers aren't sophisticated enough."
• Memory "Knowing how I would do it means I don't have to remember how I did do it."
• Superstition "I'm scared to do it differently."
• Practicality "This makes other things easier."

( via Common Lisp Tips )