The wilderness was not too hospitable, the food did not satisfy everyone; even Egypt seemed pleasant by comparison. They even said they missed the fish which they ate in Egypt chinnam – freely.
Ramban thinks fish was so plentiful that the Egyptians gave it away free, Ibn Ezra says it was not actually free but inexpensive. But the sages argue that if the Egyptians refused the Israelites straw to make bricks, why would they give them fish? No, they say: the fish was free of mitzvot.
They mean that things had changed after the Exodus and now the people had responsibilities and commandments. With the fish came a sting in the tail. There were laws of honesty: you might not steal another person’s food. There were laws of charity: others also had to have food. There were duties to God: you had to make a b’rachah.
Maybe in Egypt one was free of all this. But is it not better to be out of the house of bondage with dignity restored and a chance to reach for the stars?