I would like to say that I, Professor Minerva McGonnagal, am a studious and observant person. And during my years at Hogwarts, as a teacher and a student, I've come across one great pattern: Potters and red haired girls. I like to refer to it as The Potter Pattern, with capitalizations and all.
During my years at school there was young man in my year called Harrison Potter. He was tall and had wild black hair and dark brown eyes and glasses. He was handsome and smart and had little respect for rules. He played on the Gryffindor Quidditch team as the Keeper, and during his reign as Captain, we never lost the Cup. Many girls pined after him, but he always chased after one girl. A red head girl named Annie O'Connor. Annie was smart and petite but her size did not match her temper. For years Annie turned down Harrison, but sometime in our sixth or seventh year they began dating and as fate would have it, Annie and Harrison were married.
After we graduated and I became a teacher, I met one of my most memorable students, James Potter. He looked exactly like his father, to the T, glasses and all, but had his mothers hazel eyes. He acted much like his father, and if possible had even less regard for the rules than Harrison. James, like his father, met a red head girl, Lily Evans. Lily was a little on the short side and had a temper to match Annie O'Connor. James chased after her for years, being turned down over and over. I had a feeling that they would end up together and marry and by their seventh year I was right.
Which is why when Harry Potter arrived in my class I knew, that he would marry a Red Head. He was just like his Father and Grandfather. He looked exactly like them except he had his Mothers' brilliant green eyes. (I had a pattern within my Potter Pattern) He had little regard for the rules and excellent Quidditch skills. I searched and searched for a red head but couldn't find one. (Well there was one but he was a boy) The following year when Ginevra Weasley arrived at Hogwarts I knew they would be married one day. Why? Because of the pattern.
When Harry went out with that Ravenclaw, Cho, I knew it wouldn't last because she didn't have The Hair, and was a Ravenclaw. Potters liked strong and smart Gryffindor red heads, and Cho was a whiny Ravenclaw. That relationship was doomed from the start.
So when Ginny and Harry were together, I commended myself on a job well done. My Potter Pattern had gone through three generations, without a flaw. I was so proud of my observations.
So, during the sorting one year, I was not surprised to see another Mr. Potter, (David) with chocolate brown eyes like his mother Ginny. Eventually he was on the Gryffindor Quidditch team as a Beater and Captain. And I wasn't surprised to find that there was a small Red Head in his year named Katie Cummings who had bright blue eyes.
I have a feeling that one day they'll be married happily with a little black haired boy with blue eyes like Katie. Because my Potter Pattern is always right. And because Potters and Red Heads are meant to be, and always will.