Yosef searches for his brothers. At first, he is unsuccessful but ultimately he meets an anonymous passerby who guides him to the fateful encounter which ultimately changes the course of history.
The Midrash says that this anonymous passerby was none other than an angel.
The Ramban explains that the intent of the Midrash is to say that the passerby played a role, albeit unwittingly, in the unfolding of Divine Providence, and in that sense he is called an angel.
This insight challenges us to think about all the "angels" we have met in the course of our lives, and to realize that unexpected meetings do not always come about by chance.
There are people that we are meant to meet.