Answer:
His need to see the king so as to make whatever requests he has makes him decide to tell the king about his daughter's 'supposedly' ability to spin gold from straw.
Explanation:
In the fable of "Rumpelstiltskin" by the Brothers Grimm, the story revolves around a miller's daughter capable of spinning straw into gold. And through this story, the theme of being truthful and being responsible for one's own actions are brought into the fore.
In order to feel or be taken superior, or at least taken seriously, the miller decided to lie to the king by stating that his daughter could spin straw into gold. Greedy as the king was, the king immediately asked for the daughter to prove her father's claim, which resulted in the actual production of gold from straw.
So, the main reason or motivation for the miller to tell the king about his daughter's ability to spin gold from a straw seems to be that he wants to be taken highly or to appear superior.