When things break, it's not the actual breaking that prevents them from getting back together again. It's because a little piece gets lost - the two remaining ends couldn't fit together even if they wanted to. The whole shape has changed.

This quote is saying that when something is broken, the pieces don’t necessarily fit back together again. Even if someone wanted the pieces to fit back together, they couldn’t because something is lost in the breaking process. The shape of the object has changed and the pieces no longer match.