'Dogs are natural gamblers. If they "win" one of these exchanges once, they'll continue to place bets. Make the result inevitable until you are sure your dog isn't calculating the odds.'
Excellent information in this post from Baltimore Dog Works. I'm sure many of you who have trained with me have heard me say the same things in one form or another.
http://www.baltimoredogworks.com/blog/2015/9/7/10-rules-for-training-your-dog