why does Darcy think his proposal is correctly worded when speaking to Lizzy?

He believes himself to be doing her a favor and that he's indeed a gift for which she should be grateful............. she appreciates the compliment but isn't impressed with the sentiment.

"In vain have I struggled. It will not do. My feelings will not be repressed. You must allow me to tell you how ardently I admire and love you."

Elizabeth's astonishment was beyond expression. She stared, coloured, doubted, and was silent. This he considered sufficient encouragement; and the avowal of all that he felt, and had long felt for her, immediately followed. He spoke well; but there were feelings besides those of the heart to be detailed, and he was not more eloquent on the subject of tenderness than of pride. His sense of her inferiority -- of its being a degradation -- of the family obstacles which judgment had always opposed to inclination, were dwelt on with a warmth which seemed due to the consequence he was wounding, but was very unlikely to recommend his suit.