Ba'al Shem Tov, Noach 12/בעל שם טוב על התורה·נח י״ב

1The living creatures ran and returned. (Ezekiel 1:14)Constant pleasure becomes second nature, and is no longer pleasurable. Therefore, a person rises and falls in serving G-d, so that he should experience pleasure, which is the main aspect of worship.Toldos Yaakov Yosef, Tazria, p. 92b; Shelach, p. 139c