Likutei Moharan 169:4/ליקוטי מוהר"ן קס״ט:ד׳

14. In addition, I found the following: This corresponds to, “… then God will safeguard the Covenant for you.” For when you merit to joy, then God Himself will safeguard the brit for you. That is, He will assist you in guarding the holy brit. This is because the main blemish of the brit is through sadness, as is known.

2For the blemish of the brit comes from the known evil force which is called LiLiT because it always m’yaLeLeT (wails)—the aspect of sadness.

3Therefore, the main guarding of the brit is merited through joy. This is because through joy one merits to have God Himself assist him in guarding the holy brit, as above.