It is not so much his appearance that makes him attractive (though aesthetically, he is very pleasing.)

It is not his power (even though he is arguably the strongest being currently in existence), because as a medic-nin, she holds the power of death and life.

What pulls her to him does not even have anything to do with their team bonds (which are thicker than blood, in most cases.)

The real reason she loves him, is simply that he loves. He is the embodiment of the idea of love. He is sunshine, he is laughter, he is all that is positive. His joy is too much to contain in one body, so it overflows into everyone that comes near him (it must be contagious, Gaara even caught it.)

She can't not love him, because, really, who can deny love when it is so openly offered? When it is practically packaged for you (in bright orange, no less), how do you say no to it?

She gave denial up long ago. When Sasuke was around, she fought it, built up high dams to keep it away. After he left, at first, she pushed back hard and tried to protect herself from that hurt again, from what would happen if she lost control of her safeguards. He never stopped loving, eventually eroding her resolve away, exposing her to the current.

She does not mind it so much, really. He makes drowning well worth it.