I was inspired to write this by the debate that has taken place on various boards about the spoilers for episode 9 of Angel Season 5. In this episode, Spike tells Harmony that he decided not to take a trip to find Buffy because it would detract from his sacrifice in 'Chosen'. Many fans could not understand his reasoning. I thought I did. For me, it was all about the nature of sacrifice. I took a closer look at what Spike's sacrifice entailed.
I believe that Spike died not only because he felt it was the right thing to do but also because it involved overcoming the greatest temptation he has had to face since he fell in love with Buffy. Since he became conscious of the love he felt for Buffy in 'Out of My Mind', Spike has been on a journey of redemption. After gaining his soul, his journey is Christ-like in its imagery; the cross in 'Beneath You'; crucifixion in 'Never Leave Me'; scourging (in the form of beating and drowning) in captivity in 'Bring On the Night' and 'Showtime'; betrayal by Giles in 'Lies My Parents Told Me'; and the temptation by the First Evil in the form of Drusilla, himself and even Buffy.
The temptation in the form of Drusilla offered Spike a return to a relationship that he thought was gone forever; one that had lasted over a hundred years. A return to the "side" of Drusilla who had provided him with a destiny as a "creature of the night." In rejecting Drusilla, Spike rejects Evil and actively chooses to be on the side of Good.
The temptation in the form of himself is an even more powerful one than that of Drusilla. This is Spike's demon, the part of him that provides the strength and power, the part of him that Buffy needs to help her fight the First Evil. Spike offers to sacrifice himself at the end of Sleeper when he realises he has been killing again, even though he doesn't know why or how. More than that, he actively seeks his demon's destruction by Buffy. When she refuses, Spike sacrifices his power and strength when he asks for her help, something he said earlier (if only to himself in a madness-induced hallucination) he could never do. Realisation that the First Evil is controlling his demon motivates Spike to taunt Buffy in the hope that she will kill him in 'Never Leave Me.' It is only after Buffy pledges her faith in him that Spike is able to begin to rebuild some form of self-worth that is not rooted in the strengths of his demon. He begins to believe in himself as something more than a monster, someone who can once again be a good man. The First Evil makes use of Spike's dependence on Buffy every time it appears in her form. It is Buffy's form who goads Spike to kill the girl he meets in the street and it is Buffy's form again who tries to undermine his attempts to stay true to his desire to fight on the side of Good, when it tells Spike that Buffy doesn't value him enough to save him. (Showtime)
When Spike is finally freed from the control of the First Evil (in 'Lies My Parents Told Me'), he finds for the first time since becoming a vampire with a soul, that he has gained more power than he has lost. He has regained control of his own Free Will. He knows that he has the power of choice and he recognises that he can choose to resist temptation or give in to it.
The First Evil can, and does, make use of Free Will and Buffy inadvertently provides the First Evil with its most powerful weapon, in Chosen, when she tells Spike "I love you". Spike is about to die, willingly, because it is the right thing to do. He even accepts the method of his death, unable to fight or move, pinned in place by the power of the amulet working through his soul to seal the Hellmouth forever. This in itself is a huge sacrifice for Spike to make. He is a powerful vampire, whose joie de (un) vivre is rooted in the fight. He once told Buffy that he was willing to die to protect Dawn, that he always knew he would "go down fighting." He regained his fighting power for Buffy in Get it Done when he put on the duster "costume". Acceptance of the power of the amulet and yielding his own power is part of the sacrifice; it's what marks him as a true Champion. When Buffy tells him she loves him, he is offered the one thing he thought he never deserved; to be loved, by her. This is his final temptation.
When he denies Buffy's words of love,
Spike overcomes this last temptation. What would be the consequences had
he not done so; the Hellmouth might not have been sealed? By staying, Spike
becomes the final guardian of the Hellmouth, releasing Buffy to "go on
living". His sacrifice is at its most powerful in this, his release of
Buffy, his loss of being the beloved.