Because it is. Even their love theme is called “Chosen”!
Everything about the two of them together has been because of choices they have both made. Both of them bucked fate by simply being together. Neither of them sat there and said “you are my destiny, we are meant to be”, in fact, Spike actually says, in Crush, “This … with you, is wrong. I know it, I’m not a complete idiot.” But still they chose to stand by one another, through everything.
That is one of the major reasons that Spuffy rates so high with me. It didn’t happen because it was “supposed to”. It happened because they both made decisions and came to the realization, of their own volition, that being together was worth making sacrifices for, was worth changing for.
My OTP chose to be together, even though it would have been so much easier to stay apart. And that is beautiful to me. That is true love. That is real and powerful and amazing. To choose to be with someone, against all odds, when you know the world is completely against you … it’s the stuff of legends.