Spike always thought that the “good” chemistry between the two siblings had never been reciprocal. To have figured that out years before Twilight did due to her inherent bias was as good for his self-validation as it pained him. Because if there ever was a time to put the lid on a ‘I told you so?’ Now was probably that time. Hence, he bit his tongue, not wanting to be caught giving the wrong answer to a rhetorical question.