If you're looking for details, then head over to the manga section.

When Sorata and Arashi first meet, Sorata fixates on her while Arashi skirts around the subject of his feelings, trying to keep herself from the emotions that he offers. During the first ten or so volumes of the manga, their relationship is somewhat like that; Sorata will flirt outrageously and Arashi, with a very straightforward face, will physically abuse him *grins*. Of course, the rest of the household enjoys these antics.

As the series continues, we see Arashi open up to Sorata more and more, and Sorata himself starts to show more of a serious face. When Sorata starts to be more sincere in his efforts, namely asking Arashi about her feelings in a more staightforward way then he had before. Arashi starts reacting differently, blushing over his comments (and obviously taking him much more seriously than before). As this continues, the two's feelings begin to become deeper...

By the "beginning of the end" of the series, the two end up consumating their relationship (thus making this fangirl very happy) and Arashi leaves to speak with Hinoto. Sorata has to come to her aid and....

Why are these two so suited for each other? What exactly makes you crazy fangirls so obsessed with them? These are common questions for someone who hasn't seen the light that is Sorashi or, as the case may be, has met me. Here's the best explanation that I can offer.

Namely, their personalities are very well suited for each other. Sorata is what Arashi needs, and vice-versa. Arashi grew up in a situation where there were not a lot of personal feelings, and Sorata, in offering a chance with this, is fulfilling something that she really needed. Likewise, Sorata is the type of person attracted to people, and he needs to be needed. The two fulfill this part of each other.

Confusing, huh? ^^;; I know. That's the best explanation that I can offer on a logical level. Really, the two just make me feel all gooey inside. And there's nothing like canon Sorashi sap to put me in a good mood.