Celebrating Become A Servant

Rav Yerucham Levovitz (Daas Chachma U'mussar maamer 41,43) elaborates that the avodah of Pesach, of the seder night, is to accept upon one's self the avdus of Hashem.  The point of leaving Egypt was not to just leave Egypt, but is to become the avadim of Hashem. (He says this is part of the obligation of hakaras hatov that one owes to one that does them a favor to to do that person's bidding and the obligation of hakaras hatov to Hashem obligates us to become His avadim.)  

This is what the Midrash Tehillim (Ch, 113) says באותה הלילה שנגאלנו ויצאנו מעבדות לחירות שהיינו עבדים לפרעה וגאלתנו ונעשינו עבדים לך שנאמר הללוקה הללו עבדי ה.  The Midrash says further on that when Pharaoh told Moshe to leave already he said,  אמור הרי אתם בני חורין הרי אתם ברשותכם הרי אתם עבדים של הקב"ה. התחיל פרעה צווח ואומר לשעבר הייתם עבדי אבל עכשיו הרי אתם בני חורין הרי אתם ברשותכם והרי אתם עבדיו של הקב"ה צריכים אתם להלל לו שאתם עבדיו שנאמר הללויה הללו עבדי ה', Pharaoh had to declare not only are you free but you are not the servants of Hashem, built into the very act of freeing Klal Yisrael is the acceptance of the avdus of Hashem.  The Maharal (Gur Aryeh Bo 12:6) explains the two mitzvot of milah and dam pesach represent the badge of being an eved Hashem (milah) and an act of avdus (pesach) for that is what is necessary for going free; accepting the service of Hashem.  

The Midrash says צריכים אתם להלל לו, the acceptance of the avdus of Hashem is an impetus to sing hallel.  Normally, one who is an eved is not rejoicing but to become an eved Hashem is the greatest merit for a person.