Flashback from Exactly 50 Years Ago
One way or another, while looking at Kikyou, Sesshoumaru felt strangely nostalgic. Why he never felt that before is a mystery. All of a sudden, memories flooded back to him involving a priestess that he met fifty years ago. Why it was fifty years ago was not mentioned. After all, that was when Kikyou was with Inuyasha and she did not seem like the type to cheat on people. But what do the viewers know? They've only watched eighty seven and a half hours of Inuyasha (hopefully not in a row).
Anyways, Sesshoumaru remembered how he fell in love with the priestess. One would think that you would remember the love of your life even after fifty years; especially if that person's appearance did not change (due to some cheap and crappy resurrection that couldn't even get her a real body).
Somehow, everyone (that is, all the writers) managed to relate this event to the song Fukai Mori. Since this generation consists of mainly people who believe love can only be in a romantic sense, they figured that the first stanza of Fukai Mori obviously states that Sesshoumaru's heart was broken by a woman. It could quite possibly be a man too, but I haven't bothered to check in on the other fandoms such as Inuyasha x Sesshoumaru and Miroku x Sesshoumaru (yes, they do exist).
There was a whole story about how the two of them loved each other but were separated one way or another. The most common theory is that Sesshoumaru had to fight a war against the cat clan and left Kikyou. When he came back he found her either with Inuyasha or dead. However, the theory fails to mention why Kikyou, who 'loved' Sesshoumaru failed to remain loyal in less than a year (since Sesshoumaru met her fifty years and she was with Inuyasha fifty years ago). But again, being the kind and forgiving readers you are, you will pretend that you did not notice such a plothole and that no such plothole exists.
Kikyou, for one reason or another, did not seem to remember either. Although it would be stretching it, it could be argued that Sesshoumaru merely forgot (or got amnesia). However, during the fifty years that Kikyou was dead, she didn't remember anything, so when she was resurrected, one would assume that the memories from fifty years ago would still be clear in her mind, but no. One of the best ways to get across this is to say (as one story did) that Sesshoumaru fell in love with Kikyou, but Kikyou just thought of him as another demon, so she completely disregarded him. Surprisingly enough, no one bothered to think for a while and come up with this with the exception of one person (who is not me; I got this idea from a fanfiction). Unfortunately, this was not written to point out the reasonable fanfictions about Kikyou and Sesshoumaru. Therefore, we shall say that Kikyou and Sesshoumaru loved each other but then forgot/got amnesia.
(Fancy decorations like ) Flashback (more fancy decorations)
Sesshoumaru went to Kikyou's village to try and kill her because he was feeling bloodthirsty and fancied hunting down people that were at his own strength level. A bloodthirsty demon with common sense would've attacked weaker beings, but remember, in this story, Sesshoumaru's an idiot with an ego the size of Jupiter.
However, after ranting on and on about how he was going to go about his killing spree, he decided not to once he realized how kind a person Kikyou was since she would not kill demons that were not threatening to take the Shikon no Tama or hurt the villagers. You wouldn't think that Sesshoumaru liked goody-two-shoes female, but he does in this story. We will disregard the obvious logical reasoning that if he liked goody-two-shoes, he would've hitched up with Kagome around episode fifty or so. And if Kagome took periodical baths like her health and hygiene teacher instructed her to, there would be no scent of Inuyasha. And since Inuyasha did not mark her, one would assume that she wouldn't be "Inuyasha's woman". Besides, if Sesshoumaru hated touching anything that "belonged" to his brother, what the heck was he doing with Kikyou for a nanny? We will pretend that that is also not a plothole. To help with the pretending, I suggest to start whistling and having a completely oblivious look on your face because that cliché obviously shows complete innocence.
Now, we have gotten off the topic with our pretending to not have noticed and examined a plothole. For a multitude of reasons, Sesshoumaru decided to stay near Kikyou. One reason is that he was wounded and Kikyou, being the ever so kind priestess that was stupid enough to heal a jerk like Onigumo, healed him despite his protests (you can see that she obviously respects the rights of others).
While being healed, Sesshoumaru fell in love. So did Kikyou. As shown through the series, they're just sort of emotional and spontaneous like that. They also decided to spontaneously profess their love while doing nothing in particular except be near each other.
So they got together, had several midnight rendezvous because the villagers didn't like demons (even though they let Inuyasha use their village as his home base), and said a lot of cheesy things. After that, Sesshoumaru went to who knows where and several months later, Kikyou decided to get hitched with his half-brother. Ah, how beautiful true love is.
When Sesshoumaru got back and found Kikyou either with Inuyasha or dead, he went off on another rampage (hmm, maybe the fit he was having in the beginning of the flashback was because someone died too). This time, the strong priestess that he decided to hunt whacked him over the head with a quiver full of arrows and he got amnesia. That could have been why he forgot.
