N is for Never

He will never say it, no matter how he sometimes thinks it. He won't even let himself believe that what he thinks is real.

The words are unfair because they are lies; they are illogical and impractical and representative of far more emotional outpouring than a Decepticon is capable of. There is no such thing as a 'Con in love because to love another is to weaken one's self. There is only bonding, which provides a strength for every weakness it creates.

So he won't let himself say it, because he respects Starscream enough not to lie, and because he knows his bond-mate would never take the words as more than another example of his stupidity, his naivety. He owes Starscream too much to lessen what is real between them by voicing childish lies.