Wishing may be the definition of futility, or atleast it's most frequent manifestation.

My argument is as follows...

It's futile to wish
Wishing, it seems to me, is done with a sort of carelessness coupled with bisbelief in most cases (aside from those wishes found in fairytales). It's a declaration of a desire for something that one feels is impossible under current circumstances or, simply, something one desires to magically appear rather than doing what's necessary to attain it. So, the futility of making the wish is known in making it which makes the actual wish futile. (i.e. a thirsty and lazy individual may sit on a couch wishing for a glass of water to appear before them)

...yet wishing is done with the most sincerity in times of distress and utter hopelessness
Someone stranded in a desert may shout in agony thier wish for a glass of water. The probability of the wish coming true in a time of desperation is no greater than at any other time, yet the wish is by far more sincere. So, when a person is most likely to truelly believe (or at least hope) that thier wish has a chance of coming true the futility of the wish still stands.

As for those times that wishes come true, whether out of coincidence, doing what's necessary to attain or some divine intervention, perhaps these instances stand to show that what may seem a futile attempt or impossible desire is not always so.