The handmaiden who spoke with angels
Sarai was correcting a fundamental error that Hagar made. Once she became pregnant, as Rashi explains, she grew conceited and looked down on her mistress, claiming that Sarai is not the Mrs. Perfect everyone considers her. If she were so good, she would not have remained childless while Hagar conceived instantly is what the handmaid thought to herself. As the text says, vatekel gevirata be'eyneyha [her mistress become of less consequence in her eyes] (16:4). Here Hagar saw incorrectly and reached the wrong conclusion by assuming that what she saw defined all that is. The angelic directive to return pointed out that she erred and she must accept Sarai superiority of position and spiritual level.
For over 17 years, things seem to stabilize in Avraham's household. But when Sarah sees the danger that Yishmael poses to her son, she insists on sending him and his mother away. Hashem Himself tells Avraham to listen to all that Sarah tells him (21:12), and so he sends mother and son off the very next morning even though Yishmael is sick. What's very interesting is how this episode of Hagar's leaving contrasts with the previous episode. The first time, she ran away. Now she is sent away. Also the first time she was alone, and here she has Yishmael with her. The first time she is ordered to return by the angel, but this time, she is shown how she and her son will survive away from Avraham's household.
But what struck me is also the difference in the manner in which the angel addresses her. In the first instance, she sees the angel who speaks with (no fewer than 5 according to the view that each statement by a different angel). Here she only hears the angel call her from the Heavens (21:17). Also when she was ordered to return, she was also reassured that she would have a son to be called Yishmael ki shama Hashem el anyech [because Hashem heard her distress] (16:11). There she merited to be heard. This contrasts with what the angel tells her the second time around, ki shama Elokim el kol hana'ar [for G-d listened to the voice of the lad] (21:17). Though Hagar had cried herself, she is not credited with anything meriting G-d's response.
Even in her great decline, she merited to hear an angel, though now that she was sent away by Avraham, she no longer merited to see one. Chazal learn that at this point Hagar had actually reverted to idolatry, which is why she had to be expelled from Avraham's household. According to some, when she cried out, she was crying out to her idols. According to others, when she cried out, she was complaining that G-d's promise that her descendants would multiply appears false, for she expected her son to die. She separated herself from her sick son, saying al ereh bemos hayeled [I won't see the death of the boy] (21:16). She had willful blindness. Consequently, she needed to have her eyes opened for her. Vayifakach Elokim eth eyneyha vatera be'er mayim [G-d opened her eyes, and she saw a well of water. The well had always been there; only Hagar had failed to see it. That is the revelation for Hagar in this episode -- to realize the limits of her vision. What she was able to see was not all there was; even human who have seen angels can fall into errors of vision.