Buffy is a character torn between two identities she struggles to integrate, the lone warrior Slayer and the perky "just a girl" cheerleader. Spike is a character who bridges the divide between the Other (the shadow, demons) and the Self (the light, humanity); a demon who retains a sense of humanity upon turning into a vampire, the ability to love, and through loving Buffy and his own determination, he fights back for his soul, reuniting both Other and Self within him. He is a guiding star for Buffy, who fears she's losing touch with her own humanity and turning hard as stone because she's the Slayer. Spike helps Buffy reconnect to her humanity. Spike who "like[s] this world" and who lives every moment to the fullest. Spike who's so emotionally open because he's incapable of existing without wearing his heart on his sleeve.
Look at this GIF.
Dawn is Buffy's humanity. Spike is Buffy's shadow. Buffy should be standing in between Spike and Dawn, being torn in opposing directions, yet it's Spike who connects Buffy to Dawn, to her humanity. ("Life's not a song. Life isn't bliss. Life is just this. It's living. You have to go on living. So one of us is living.")
By mid Season 6, Buffy's uses her connection to Spike to draw away from her friends and she loses herself in the darkness (just as Spike loses himself in the darkness where before he was moving towards good through Buffy's example). She eventually leaves him in his dark crypt and walks into the sunlight. marketchippie described this moment when Buffy rejects Spike in "As You Were" as a call of seduction--"I'm sorry, William"--and I think she's right. Buffy can't follow Spike into the darkness to be with him because "it's killing [her]", so she returns to the light and he follows her (just as he's done for most of Season 5 and early Season 6--Spike following Buffy into the sun, no matter how dangerous it is to his vampiric nature). Spike crosses the divide, fights back to win his soul, and joins her in this state of duality where they both struggle to integrate both demon and soul spirit within them. Their journeys mirror each other. Just as they fall into darkness together, they ascend into the light together. They build each other up. They make each other stronger. They make each other whole. They make each other one.
This is why Buffy loving Spike (as many doubt) becomes essential to the narrative--Buffy must fully embrace the Other and the Self in order to find harmony. She finds this harmony by loving the darkness. The effulgence Spike experiences in "Chosen" is an ecstatic moment of love and unity. That's why Buffy can smile in the end, because she and Spike healed each other. When I think about the last word Buffy speaks in the series, "Spike," I think about the way she looked at him in awe. I think about their two hands clasped tight, fingers intertwined as they burst into flames.
In that moment they are one. They are complete. They are each other's mirrors that reflect light. Without a soul, Spike is a creature who cannot fully reflect light into a mirror. With his soul (and the amulet), he reflects the sun. And so Buffy basks in what's real, overwhelmed and trembling in awe. She clasps his hand and tells him she loves him. Spike's effulgence is Buffy's effulgence. She gets her fire back, her ability to love, her unity of self, through learning to love and embrace Spike in her heart.
My ship. The narrative perfection of my ship. ♥