The Alter Rebbe was in the village of Piena, where he passed away on 24 Teves. Among the things he said was the following in the name of his Rebbe, the Maggid of Mezritch. A person is judged from Heaven based on the way he behaves on earth.
The Tzemah Tzedek was also with the Alter Rebbe when he repeated this. He said that the Alter Rebbe prevented him from singing a melancholy type of niggun during Maariv because if he is sad here on earth, he will cause the same in Heaven. This isn’t good for him.
A point of clarity: don’t equate a Lubavitch dveikus nigun with a sad niggun! It seems the Alter Rebbe is talking about a sad niggun that makes one more depressed not more frustrated, which leads to positive pro-action.