The evil inclination strives to make a person sad, thereby preventing him from serving God. One must understand that the evil inclination is deceiving him, and one must strive to the best of his ability to avoid being sad.

The evil inclination sometimes misleads a person into thinking that he has sinned:

The evil inclination sometimes misleads a person by telling him that he has committed a terrible sin, when in reality he merely ignored a stringency or did something that is not a transgression at all. The evil inclination’s intent is to cause him to be sad and, due to his sadness, to cease serving the Creator, may He be blessed.

It is critical for one’s service of God to avoid sadness:

A person must understand this deceit and tell the evil inclination: “I am not paying attention to the stringency you are telling me about, for your intention is to stop me from serving Him, may He be blessed. You are speaking falsehood. And even if it is a bit of a sin, my Creator will be more satisfied with me ignoring that stringency you are telling me about, causing me sadness while serving Him. On the contrary, I will serve Him with joy, which is an important principle. For my service is not for my own sake but to give Him satisfaction, may He be blessed. Therefore, even if I do not pay attention to the stringency you tell me about, my Creator will not be strict with me about it, because the only reason I ignore it is to avoid ceasing to serve Him. How can I cease serving Him even for a moment?” This is a great principle in the service of God, to take care [to avoid] sadness as much as possible. (Ba’al Shem Tov, as cited in Tzava’at HaRivash 44)