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That you would rather add than subtract
It's as simple as you think. It's saying, like a mathematical term, that you are willing to be with him (like being another part in his life) like adding 1 to his life to show that both of them equal 2.
It means that you'd also rather be in his life rather not be in it. In a
clearer statement, it means that if you'd subtract, you are not going
to be with him or just stay as friends etc. While adding means that
you'd be willing to stay with him and being more than "just friends".