"No man steps into the same river twice, for it is not the same river, and he is not the same man."
The Buddhist law of impermanence (Anitya) states that all things in creation arise only to pass away. The only constant thing, is change. Feelings come and go with great rapidity. They change moment by moment. Even our physical bodies are changing with the constant regeneration of cells. The message I take from this into my life is that all things - good and bad, end.
When we reject our negative experiences or try to hold on to our positive experiences we are in conflict with the law of impermanence.
Our suffering will end.
Our joy will end.
There is liberation in this. We can then simply experience them (enjoying the good and grieving the bad), knowing that they are impermanent states. The only constant is change.