Although Mr. Giovanelli is not really a gentleman, according to the rules of polite society, he is a friend to Daisy. He does not judge her or try to convince her to behave in a certain way. He does not push her around, but he does not look after her best interests either. He knows that what she is doing can damage her reputation and he also knows that Daisy will be vulnerable to the Roman fever, yet he does not protect her. Maybe that is why Daisy liked him so much.
From my view, I think that Mr. Giovanelli is very flattered to have Daisy's attention. He is a wannabee, putting on airs with his fancy clothes and flowers in his buttonhole, he is suave and slick. He enjoys being in Daisy's company.
Daisy was a girl who knowingly took risks so that she could experience life. Perhaps she particularly chose Mr. Giovanelli, because she knew that she could do what she wanted with him and he enjoyed being in her company.