91 Homewrecker alert

The rhythmic hum of the engine filled the truck's interior as Sky skillfully navigated the city streets. Rachel occupied the passenger seat, engaged in an animated conversation with Sky about their day, while Chris and Rain sat side by side in the backseat. Chris's eyes occasionally met Sky's in the rearview mirror, and for some reason, Chris was always the first to look away. The stare made him uncomfortable since Rain was sitting very close to him. 

Why did he feel like a cheating partner when he wasn't cheating and, most especially, when he had no partner?