Reasons
Willow and Xander have been friends since early childhood. They know the other better than they know anyone else and probably better than anyone else will ever know them.
Through the years of their friendship their feelings for each other have deepened and although they don't want to admit it, not even to themselves they love each other. They have a deep connection which links them and some even believe this connection is the connection of soul mates.
Throughout the six seasons we've seen their relationship change, we've had moments where both Xander and Willow have revealed their deepest feelings. In 'Becoming Part Two', Xander reveals that he loves Willow, whilst she is in a coma. In the 'Zeppo' Willow tells Xander how she feels before the end of the world can destroy their lives.
The fluke in 'Homecoming' brought to a head their feelings, but because of their other relationships their liaisons were a secret and when it was finally revealed in 'Lover's Walk' their friendship was damaged by the revelation.
Willow and Xander are destined to be together, they have a connection which will inextricably link them forever. Although their friendship at the moment may be strained the end of season six has seen a dramatic change in their relationship. Xander supported Willow through her addiction to magic. He was also the only one to reach her when she was left devastated and seeking revenge in the season six finale. Xander's declaration of love not only saved the world but also themselves. Their love will defeat all obstacles and they are destined to be together.
