Energy flows from where attention goes.
If there's something that I wouldn't like to happen again in my life, then I shouldn't bring it back by giving it attention. (Much less have others feeding it either!) That's why some things are better not spread.
We have to learn to let go, and use that attention to strengthen the attraction of good things instead. As that quote says: Whatever I think about with more intensity will grow stronger in my life. Whatever I truly leave behind will weaken and disappear.