Someone once told me that the most powerful word in the English language is the word "because." Reading Influence: The Psychology of Persuasion by Robert B. Cialdini, I finally stumbled upon evidence for the statement. It seems it's based on work done by Harvard social psychologist Ellen Langer.
In Cialdini's words: "A well-known principle of human behavior says that when we ask someone to do us a favor we will be more successful if we provide a reason. People simply like to have reasons for what they do. Langer demonstrated this unsurprising fact by asking a small favor of people waiting in line to use a library copying machine: Excuse me, I have five pages. May I use the Xerox machine because I'm in a rush? The effectiveness of this request-plus-reason was nearly total: Ninety-four percent of those asked let her skip ahead of them in line. Compare this success rate to the results when she made the request only: Excuse me, I have five pages. May use the Xerox machine? Under those circumstances, only 60 percent of those asked complied."
He goes on to say one might think that the difference between the two results was the additional information she offered - that she was in a rush, but Langer tried a third type of request that puts all the power in the word "because."
"Instead of including a real reason for compliance, Langer's third type of request used the word "because" and then, adding nothing new, merely restated the obvious: Excuse me, I have five pages. May I use the Xerox machine because I have to make some copies? The result was that once again nearly all (93 percent) agreed, even though no real reason, no new information, was added to justify their compliance."
The word "because" according to Cialdini triggers something in us as humans that makes us comply. That's why you should use the word because - because it works.