Aharon your brother... he comes out to meet you, and he will see you and be happy in his heart. (Shemot 4:14)
Why does it matter that Aharon was "happy in his heart"?
Aharon was the oldest, firstborn brother, and here he was called upon to take a subordinate role to Moshe. Elsewhere in the Torah, older brothers typically get angry and jealous when their firstborn role is taken away from them.
The exile in Egypt started when Yosef's brothers sold him into slavery rather than accepting a subordinate role to him. Fittingly, it ended when Aharon was able to graciously and peacefully accept that Moshe had been chosen as the leader, and work together with Moshe to get the Israelites freed.