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My interpretation is David coped with his grief and loneliness by living Cliff’s life. Now that he’s cut off, he’s alone again. Even though there is still Cliff, other than the fact that they had a fallout, David said Cliff doesn’t know what it’s like to be him. The only way for David to not feel alone again is get Cliff to experience losing everything as he did, and to suffer as he does, so he has a companion who he can relate to and understand his pain to spend the next four years with, and they either live through the rest of the mission together or die together. Misery loves company.
Cliff’s kid died thinking his father was murdering him. It is not the same, I’d doom tbe mission before I’d take that chair.
Not saying it’s rational. That’s my interpretation from what David said. He was driven mad with grief and isolation. He doesn’t even care if Cliff would doom the mission. They either suffer together or die together. He wanted to have something no matter how twisted it is.