As much as Starscream hates to admit it, this storm is far too dangerous to fly in. So, he has to walk on the ground, like a peasant.
At least he's not alone though. You're there with him, cruising along in your own vehicle mode and matching his pace. You'd learned pretty quickly that appealing to his ego was the best way of keeping him in a good mood, so throughout the entire way you play it safe and ask about his paintjob, and his top marks at the Cybertronian Flight Academy.
Everything's fine, up until the two of you reach a beach. You're on the way to an island, and the only bridge is flooded, making it impossible for you to drive over. Starscream himself can simply wade through the water, but you're not as big or heavy as he is, you'd probably just get swept away by the current.
There's really only one solution.
"You want me to what?!" Starscream asks incredulously, "I'm not going to carry you on my shoulders. That's humiliating! Just find your own way across."
He huffs, stepping into the deeper parts of the water and immediately almost getting swept away. It's then that Starscream weighs his options. He could risk getting swept away, or he could help you across, earning a favour from you and adding to his own weight and helping to stay put.
After a few minutes of you begging and him pretending to get more and more annoyed, Starscream finally throws his servos up in the air, scowling. "Fine! I'll help you across, but you owe me one. And don't you dare squirm, or I'll drop you," he warns lowly, leaning down a bit and waiting for you to hop onto his shoulders.
"And, if you scratch my paintjob, you'll be paying for it. And don't touch my wings, or I'll have you disassembled!"