Amber Norris posts lots of excellent information on writing. Here's the start of her post on Reason and Motive in writing. Follow the link to read the rest of the article. I'm sure you'll want to follow Amber's blog regularly.
Reason and Motive. Both are important to a successful story, if not the most important things. You may think they are the same as they are often used interchangeably, but they're not. Reason, by definition, is a justification or explanation for a belief or action of a person. Motive is the driving goal/objective that causes a person to act in a certain way and/or do a certain thing. EXAMPLE:
The King of Mars declares war on Venus to set fear [MOTIVE-"GOAL"] into the hearts of the Venusians because the residents of Venus tease them too much [REASON-"BELIEF"] .
See? The reason gives justification to the action which gives purpose to the motive. People can have all kinds of reasons for believe things and never develop a motive to do something about it. But, most of the time, people working for something have a reason why they're motivated towards it.
"So, where am I going with this?" you're probably asking.
Read the rest of the article at Science Fiction Adventures - A.R. Norris Blog