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This isn't what Sam needs, replacing one addiction with another, but Castiel can't bring himself to stop making the offer. If Dean ever notices Castiel has his excuse worked out; letting Sam drink his blood is preferable to demon blood, this way they can keep an eye on him and stop things getting out of hand. He'll tell Dean it's because he doesn't want Lucifer to win, and purging Sam's body of demon blood will make that more difficult. But that's all they are, excuses.
Castiel is selfish. He never used to be, not until he started spending time with the Winchesters. Not until he started to fall. But he is now, he's selfish and arrogant and proud, everything an angel shouldn't be. And when Sam looks at him with worship in his eyes, with a desire that goes deeper than the simple craving for blood, Castiel knows his real reasons. He wants Sam to rely on him in a way he does no one else. He wants to be Sam's strength and his weakness.
He wants Sam.