I did a simple google search on “Why Harry and Ginny?”
The first thing that came up was <http://hgnetwork.co.uk/couple.php>

That leads to: “Past & Present” interactions between the two of them in <http://hgnetwork.co.uk/canon_pastpresent.php>; and
Opinions of characters in the books – especially Ron – in <http://hgnetwork.co.uk/canon.php>; and
JKR’s comments in <http://hgnetwork.co.uk/interviews.php>…
Oh, just go and read it.