Well, I'm assuming you know this is about picking up women... not so much LTR's... however, if it were me, I wouldn't answer.
Its none of her concern why I love her, if she asks again I'd follow up with 'It shouldn't matter why I do.'
Essentially she is compliment fishing and testing you at the same time. Keep your frame. Its like if some girl came up and said 'Do you think I look hot?'... most guys go 'Yea really hot!' and then she runs off feeling very good about herself. Now if you said 'Well, I definitely like the earings.' It would be more effective in keeping her attention because you've given a sneaky compliment+neg mix.