The Torah was wary of the employer-employee relationship. Almost by definition, an employee is not much different than a slave; both are told what to do, when to do it and how it is to be done. “For [all of] you are slaves unto Me”. As our Sages teach, we are not meant to be slaves of slaves.
The aseret hadibrot present two very different reasons why we are to keep Shabbat. In parshat Yitro, it is “because in six days, G-d made the heaven and the earth, the sea and all that is in it; and He rested on the seventh day” (Shemot 20:11). Forty years later, when Moshe recounts them to the children of those who were at Sinai, we keep Shabbat so that “you shall remember that you were slaves in the land of Egypt, and G-d took you out from there with a strong hand and an outstretched arm” (Devarim 5:15). The Jew leaving Egypt needed no reminder of his days in slavery.
While there are various themes that run throughout Sefer Shemot, that of slavery is perhaps the central one. This notion is presented, with differing foci, throughout the book and actually carries over to the remaining books of the Chumash.
"And they did not listen to Moshe, from short spirit and hard work" (7:9). Moshe had a daunting dual task before him. Not only did he need to demonstrate to Pharaoh that he must free his slaves, he needed to convince the Jewish people that they would be better off following him into the desert. And the latter was a prerequisite for the former.