It was exactly because she would do the same that he had no remorse is hammering down her defenses before they were even fully rebuilt. And she only proved him right in that approach when he felt her teeth pressing into his neck sharper than he had done with the tip of her ears. That was fine by him, he had no shame and he could take much more of that than she could. So letting her have that advantage over him was more than fine with Natsu. IT was one of those ways that Lucy could really make him purr or growl or both.