This was the second time he did this. He did this in Egypt too and the king kicked him out because of it. It looks like the lesson here is to trust God instead of our own devices or schemes, any other way and we just get a great big ol mess. We see the same with Sarah and her maidservant. Instead of just trusting in the Lord's promise to be fulfilled, they resorted to Abraham hooking up with the maidservant and as expected it lead to a whole bunch of drama. Sarah started to treat Hagar harshly and she fled.