Your brain is lazy. Don't take it personally, my brain's also lazy. Everyone's brain is lazy. It's how the human brain is built. We're what scientists call cognitive misers. And this is a good thing.
Essentially, if you look back at the evolution of the human brain, we had to be just intelligent enough — just good enough at making decisions to survive and spread our genes to the next generation — but no better. So you wouldn't expect the human brain to be a perfect reasoner or a perfect decision maker. You would expect it to be good enough most of the time. And that's what we are.
So one way that manifests itself is when something happens we reach for the first explanation that occurs to us, and we generally stop there. And most of the time that's good enough. We move on.
But if you want to improve on what evolution has given you, especially in cases where maybe the stakes are higher, you can push past your cognitive miserliness and push past your explanation freeze and look for other explanations, some of which might be better than the first one you thought of.