If there is anything Starscream doesn't want to lose, it's his head, but he can't do much in the position he's in. He might as well push Tarn to the edge. People tend to make mistakes that way. And mistakes are something Starscream can use.

"You might want to work on your approach. I think Megatron still has you beat in the threats department. But I guess I can't expect much from a certified fanboy."

Starscream almost made a remark about how irresistible he is and that's why Tarn is getting all close. But knowing his luck that might actually be the case and just the thought was revolting. Ugh. Tarn cooties.