Someone once honored the Meor Einayim zy"a to serve as sandak at his son's bris. The Meor Einayim agreed to come, but only on condition that the father arrange a nice seudah after the bris. The father replied that he would love to, but he had no money to pay for it. The Rebbe told him not to worry and gave him some money to pay for the seudah.
During the seudah, the Me'or Einayim said, "Allow me to explain why I made such a big deal about insisting that there be a seudah. The name of the satan who tries to put a stop to anything good in the word is 'Samael'. His name is an acronym for the words: 'Seudas mitzvah ain la'asos. Seudas maseches ain la'ason. Seudas milah ain la'asos. Do not make a seudas mitzvah. Do not make a seudah for a siyum on a Masechta. Do not make a seudah for a milah.' He wants to stop people from making a seudah for a bris, but we won't let him do it!"
The Arvei Nachal, who was a student of the Me'or Einayim, was present at the bris and said, "The malach Michoel had a big victory over Samael. The satan takes great pride in the work he does and claims that he causes many people to transgress the mitzvos of the Torah. But Michoel tells him, 'It may be true that Yidden do some aveiros at times, but it only happens inadvertently. The proof of this is that no one makes a festive meal and invites all his friends to celebrate with him after he does an aveirah. No one in history has ever done that. But when a Jew performs a mitzvah, such as making a bris for his son or finishing a Masechta, he is so happy that he invites his friends to celebrate with him at a festive meal. This proves that the mitzvos are done with a full heart, while aveiros are only done accidentally."