UPDATE: As Jason Zarri pointed in the comments, there is possible ambiguity between sufficient reason and necessary reason when one talks about reasons. I had in my mind defense of the principle of sufficient reason when writing this post, so ‘reason’ should be read as ‘sufficient reason’ throughout the post…(Or maybe not, I just reread the post, and I don’t think, one can change ‘reason’ with ‘sufficient reason’. Grrrr…)
Something along these lines…
1) There can’t be a reason why some specific thing happens with a reason and other without a reason, as there can’t be a reason why something happens without reason.
2) But that there is no reason why a thing happened with a reason rather then no reason, is also a contradiction, as it says that both there was a reason, and no reason for it happening.
Hence, it can’t be that some things happen for a reason, and some without a reason.
a) There is a reason for everything that happens
b) There isn’t a reason for anything that happens
From there we choose an example of a thing that happened for a reason:
Sometimes, the reason I eat is that I’m hungry, hence (b) is false.
Sometimes, the reason why pool balls start to move is that they are hit by another pool ball, hence (b) is false.
Hence, there is a reason for everything that happens.
Note: One application would be that there is a reason why the specific measurement of some variable in quantum system gave the specific result.