The Gemara says if one intended to perform a mitzvah and was prevented from doing so, he is credited as if he had actually fulfilled the mitzvah. This seems to be a bit difficult in light of the Gemara which states (Kiddushin 40a) that one is never held accountable for intending to transgress an aveira unless he actually carries out his intentions. What is the difference between a mitzvah and an aveira?
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Since the core essence of a Jew is his commitment to Hashem and his commandments, we are certain he would have carried out his intentions to perform the Mitzvah given the opportunity. He is therefore credited with the Mitzvah on account of his intention. On the other hand, a transgression is not his natural tendency, therefore it is not considered done unless he actually violates.
Saw an answer as well that says the reason for this is that everyone is presumed to be a tzadik. We know that if it was not for the unavoidable circumstance, he would have done that mitzva or good intention. That is why he is is credited even if he does not do it. This can show why its only attributed to good deeds, where bad is not. – (saw in footnote from Daf vav in Artscroll – #27)