Wherefore?
Obviously, it could only be because he thought that she was flirting with him.

In other words, Freeman was jealous; and to be jealous is to love.

Now, Grace was so constituted that, though she did not like to play second fiddle herself, yet she had no objection to monopolizing all the members of the male species who might happen, at a given moment, to be in sight.
She had, consequently, already forgiven Freeman for his apparent unfaithfulness to her, by reason of his manifest jealousy of Don Miguel.
As a matter of fact, he was not jealous, and he was unfaithful; but fate had decreed that there should be, for the moment, a game of cross-purposes; and the decrees of fate are incorrigible.
"I had no idea you were so savage," she said, softly.
"I'm not savage," replied Freeman.

"I am bored." "Well, I don't know as I can blame you," said Grace, still more softly: she fancied he was referring to Miriam.

"I don't much like Spanish mixtures myself." "One has to take what one can get," said Freeman, referring to Don Miguel.
"But it's all right now," rejoined she, meaning that Freeman and herself were reconciled after their quarrel.
"If you are satisfied, I am," observed Freeman, too indifferent to care what she meant.
"Only, you mustn't take that poor young man too seriously," she went on: "these Mexicans are absurdly demonstrative, but they don't mean anything." "He won't, if he values his skin," said Freeman, meaning that if Don Miguel attempted to interfere between himself and Miriam he would wring his neck.
"He won't, I promise you," said Grace, sparkling with pleasure.
"I don't quite see how you can help it," returned Freeman.
"I should hope I could manage a creature like that!" murmured she, smiling.
"Well," said Freeman, after a pause,--for Grace's seeming change of attitude puzzled him a little,--"I'm glad you look at it that way.