Musar על שבת 60:5

The Divine Presence does not rest upon any one unless there is joy (Shabbath 30b). And all of the prophets did not prophesy at such times as they desired, but they would prepare their minds and sit joyous and glad of heart and then prophesy, for Prophecy does not dwell where idleness or sadness is found, but only where joy is. Therefore, the pupils of the Prophets would place before them lyre, drum and harp and sought by their means the gift of Prophecy, as it is written "And it came to pass, when the minstrel played, that the hand of the Lord came upon him" (II Kings 3:15).

JOY: The second [branch of love of G-d] is joy, it is a great, essential principal in serving G-d. This is what David exhorted us saying: "Serve G-d with joy, come before Him with song" (Tehilim 100:2), and "the righteous will rejoice, they will exult before God and delight with joy" (Tehilim 68:4). And our sages said: "the Divine presence rests on a person only through his rejoicing in a mitzva" (Shabbat 30b). On the aforementioned verse: "Serve G-d with joy", our sages said in a Midrash (Shocher Tov, Tehilim 100): "Rabbi Abahu says: 'when you stand to pray, your heart should rejoice, for you are praying to the Almighty of whom there is none like Him'".

The detriments to Chasidut are distractions and worries. For when one's mind is distracted and dispersed with its worries and occupations, it is impossible to turn to this contemplation; and without this contemplation, he will not attain Chasidut. And even if he has already attained it, the distractions force his mind (to attend to them) and confound it, not allowing him to strengthen in fear and love and other matters I mentioned pertaining to Chasidut. Therefore, our sages, of blessed memory, said: "the divine presence does not rest through sadness..." (Shabbat 30b).