Mooderino over at the Moody Writing blog discusses great ways to throw obstacles in the way of your characters. Conveniently responding with 'yes' to a character's wishes seems convenient, but similarly:
[...] being contrary just for the sake of it is not particularly interesting. Refusal to cooperate because sometimes people aren’t in the mood to help is more a delaying tactic than a well thought narrative approach.
A good list of examples in the original article here, with a reminder what the ultimate purpose of conflict in your stories is: http://moodywriting.blogspot.com.au/2013/06/the-power-of-yes-to-ruin-story.html