To not harbour resentment, anger, hate, jealousy, or other poisoners of the soul.
I've mentioned a fair few times before on here about how emotions self-perpetuate. If you haven't heard it before; essentially, all emotions want more of the said emotion. So the more you dwell on it, the bigger it grows, the less room you have for other emotions. Particularly less room for opposite emotions. This is true of both positive and negative emotions; so it can either be a great thing, or a bad thing. If you're dwelling on negative emotions - then you can easily get depressed, or much worse. It poisons your soul. If you dwell on positive emotions - then the negative emotions don't have any room to grow.
Now, that doesn't mean that when negative emotions legitimately arise for a proper reason, you act as if they're not there. You don't just ignore them and act like it's all fine and dandy. But when things go badly, you need to make sure that it's not all negative. Find that positive - if there's not one there, make one, put one there. We can always add a bit of positive. Often, having someone else there helps, a lot.
To me, this really matters, because I don't want to be a negative person. I know what wallowing in bad emotions can do, and it drowns you. It really does. So for me, harbouring instead hope, peace, joy, and love - that's really important. It's very reminiscent of Philippians 4:8 - "Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable--if anything is excellent or praiseworthy--think about such things."