There had been a time, once, before Earth, when Sunstreaker thought him and his brother were an item. Two peas in a pod, tank and engine, spark and processor.
Once Earth was but a speck of blue in his vision, Sunstreaker realized, with a shame that eats him even now, that it is-- was-- a lot more complicated than that.
Sunstreaker had been an awful brother. Not what Sideswipe wanted, nor what he needed. Sunstreaker had always been particularly bad at interpreting his brother's emotions, always trying to guise selfishness with selflessness. He had tried so, so hard to be a commander and a brother at the same time, at all times-- never putting a line between the two, never attempting to see how that had made his brother-- his little brother- feel.
Even now, with Sideswipe gone, he still does it. Still thinking of all the wrong things he did, all the horrible ways he'd gone about being someone Sideswipe should look up to, never someone that could have been seen as an equal.
( There will be a moment, some hours after he's had a nice breezy walk with Bob, where Sunstreaker will sit down beside a memoir made to his brother, something that should resemble him in life. Then, once he has placed a cup of energon beside him beyond the one already at his hand, Sunstreaker will tell his brother about his day, and tell him of all the things he wishes they could see together.
But this does not fit well with Sunstreaker's narrative of his own life. It does not fit well with how selfish he wishes he truly was. )