Why does Rāma say he will accept exile in the Dandaka forest?

He says fulfilling the king’s and a parent’s command fulfills dharma

Rama lifted his mother with his hands and said, “My father’s name is renowned for the steadfastness of his words. Would you rather that he spoke false? … I am thrice blessed, to make my brother the King, to carry out my father’s command, and to live in the forests. Do not let your heart grieve.”