Part 1

Sometimes a witch or wizards magical signature is so completely incompatible with another that they repel one another like magnets. On the other hand, if two magical signatures mesh well together, well there are no stronger relationships in all the world. In a sample of a thousand people, the average witch or wizard will be slightly repelled by four or five people and strongly repelled by only one, at the most. The opposite is true for attraction.
But Harry Potter can't kiss anyone at all.