Willow wasn't entirely sure when her love for Tara had begun. There had been a connection from the beginning, though--even back when Tara had been nothing more to her than the painfully shy, stuttering girl undervalued and overlooked by a catty and pretentious Wiccan group.
During that first meeting she attended, though, it definitely hadn't been attraction that Willow had felt toward the other woman. It had been pity, mostly, and a bit of righteous indignation on her behalf. (And more than a hint of discomfort to realize just how much of her high school self she saw in this outcast stranger.)
Later, when they were fending off the Gentlemen, the connection had solidified in a single, powerful instant of hand-holding, shared magic, and triumph. It still hadn't been sexual or romantic at that point, but Willow had been sure immediately that this other witch was a person she needed in her life.
So the beginning must have happened sometime later. It couldn't have been when Oz came back; feelings had started turning rosy long before that. Perhaps, she thinks, it had been when Faith woke up. Willow would never forget what Tara had said to her.
I am, you know. Yours.
There had been warm and fuzzy feelings before that, she was fairly sure, but perhaps Tara's confession had been the dividing line between friendship and something more. The point of no return, in an emotional sense. The first time Willow realized she might actually like who she was becoming.
So, she supposed, it was fair to call that the beginning. Tara had stolen her heart with five little words of devotion, and Willow had been lost.
And now that they'd reached the bloody, bitter end, she knew surer than breathing that there was absolutely no going back.