“The has kept me from having children. Go, sleep with my slave; perhaps I can build a family through her.”Genesis 16:2
God has promised a son, but that was so long ago that it looks impossible. So Sarah (and Abraham) try to take matters into their own hands by impregnating Hagar. This goes badly but God remains gracious, both to Hagar and to Sarah and Abraham.
God doesn't work to my timescale (He works to a better one), so I often try to take matters into my own hands. Trying to obtain God's promises by disobeying God will always end badly. I've done this many times, so it is good news that he continues to be gracious.
Every day, I think I can plan a better route to my happiness than the one God provides. The ends do not justify the means. God's route to my joy is not only the best way there, it is the only way there.