These things happen is just another way to say Uketamo. When something happens that is out of your control, a broken cup, burnt toast, a scratch on a car, or something more serious, ‘these things happen’ is a reasonable response, not only for yourself, but for other people who will have to live with the consequences.
Because, these things do happen. Are you going to worry about going back in the past and changing them so they don’t happen? Are you going to try and fix them, and maybe make things worse? Of course you should take precautions so that these things do not happen in future, but once they do happen, well, these things happen.