(312) Megastarop concept: the Cybertronian civil war has been over for a century. After the initial declaration of peace, and one much-photographed, significant handshake with Megatron, Optimus Prime has withdrawn from society.
He avoids a lot of people now, including dear friends, and rarely goes out. In part, it's because Optimus has had his fill of conflict he could not escape, and now even the small disagreements seem like too much work. But in part, he is just tired. Physically. Emotionally. Existentially.
The only messages he returns anymore are Ratchet's, which is probably why he isn't even aware that Starscream is trying to get in contact... until he kicks in the window. Very insincerely, Starscream claims he's performing "a welfare check," since Optimus can't be bothered to answer his emails, his phone, or his door.
Starscream, of course, fancies himself a wit. Given no incentive to stop talking and a passive audience, his wit runs very long indeed. But his real problem, when he pauses in his soliloquy to mention it, is eventually revealed: something is very wrong with Megatron, and he has come to get Optimus Prime so he can fix it.
He's the only one who can, apparently.