http://carminedimascio.com/2014/04/ BEFOREHAND BLESSINGS
[89a middle] Gemarah: Witnesses cannot testify that a wedding took place if they only knew it was planned, since there is a possibility that the woman might not agree at the last minute.
Rayvid: This is why the Bracha of marriage is done only after the act [because if it was said beforehand and the marriage does not take place, it would have been a Bracha Levatala (in vain)]. R’ Rosner: Whenever a Mitzvah is contingent upon others acceptance, we do not precede it with a Bracha.
i want to buy Dilantin D-E-ep Thoughts: Hashem continually creates and runs the world with ‘expected patterns,’ and we are required to live our lives using these supposed constants [which is why we do make blessings on inanimate objects (ie: taking the four species on Sukkos)]. But, just as other people have Free Will to choose to be part of our endeavours, we must know that G-d always has the ability to ‘break’ “natural” laws. Even though we do not get to see open irregularities anymore, we must search for Hashem in the subtleties of ‘miraculous nature.’