Why does Theseus urge Hermia to reconsider her decision to marry Lysander?

Hermia's father Egeus has made it perfectly clear that he wants his daughter to marry Demetrius. But Hermia's not interested; she loves Lysander and is determined to follow her heart, even going so far as to elope with her lover in the forest in order to escape her father's control. But before she runs off, Duke Theseus of Athens tries to persuade Hermia not to defy her father. He tells her that her father should be "as a god." In other words, Hermia should obey and respect her father as if he were one of the immortals.
Just before Theseus's intervention Egeus engages in an explosive rant in which he castigates his daughter for her disobedience and insists on his right as a father to do whatever he likes with her. Theseus's tone is a good deal more measured and reasonable, but his message is essentially the same: Hermia must obey her father's will, marry Demetrius, and forget all about Lysander. He tries to persuade Hermia that this is in her best interests, not least because she could end up being either confined to a monastery, or even worse, executed for her brazen defiance.