When a person is happy the entire day, it becomes very easy for him to dedicate an hour where he can break his heart and in front of Hashem. However when a person is sad and depressed, heaven forbid, then it is very difficult for him to do Hisbodedute and talk to Hashem.
(Sichos Haran 20)
Tzadik Yesod Olam
Rebbe Nachman of Breslov on Shmiras Habris