I stand by "everyhting happens for a reason" and tend to believe in fate / karma to a certain extent.
Of course the logical person in me knows that things are generally random, but as an optimist I will look for the good in everything. I agree with AgentZigzag that it helps me to see balance.
I also think optimists who think like this are generally much more relaxed and take things in their stride, allowing them to make the most of good opportunities, even in a bad situation.
Shit Happens is similar in that people who say it take things in their stride, but are more cynical and less likely to look for the good opportunities in a bad situation.
It ties very much into the lucky / unlucky debate. There was a really interesting 10 year study done by Richard Wiseman on luck.
This one of his conclusions, I find particularly illuminating (and true):
The differences between the lucky and unlucky people were striking. Lucky people tend to imagine spontaneously how the bad luck they encounter could have been worse and, in doing so, they feel much better about themselves and their lives. This, in turn, helps keep their expectations about the future high, and, increases the likelihood of them continuing to live a lucky life.