But, as a philosophy, I can't fault it. You can look at it as a simplified version of Nietzsche's notion of Amor fati (love of fate) wherein you accept every occurance in your life as positive and necessary. Or in Judaism, the saying "gam zeh letovah" ("this too is for the best").
i think this is precisely why it bugs me so much because i don't believe in fate at all and believe things happen because of chance and/or your own actions. like bassie said, this saying is a little too close to "it's God's will"
the other part that bothers me is that when it is said, it reeks of lazy thinking (or none at all) or is used as a cop out excuse for taking no accountability for your own actions, (i.e. well if everything happens for a reason, i really don't need to put any effort into examining this)
for those reasons above, i have always been irked by the shallowness of the saying.
I guess I'm looking at it from a different perspective. I don't believe in the will of God or predetermined fate either, so that's not really what I was trying to say. Nietzsche's quote is "Amor fati - Love your fate, which is in fact your life." So as you go through life, instead of looking at things that happen to you as "good" or "bad" things imposed by circumstance or outside forces, embrace it all as
your life, for better or worse.
So on the contrary, instead of giving up and floating through life as if everything is out of your own control, you can look at every experience as a positive and necessary part of your life, and therefore within your own control, and use that to craft your own future.
I doubt people think that much about saying "Everything happens for a reason"; I think it's just a thing people say when they don't know what to say. But that's how I see it.