The phrase “on your left” is often used in the context of running or cycling. It’s a way of indicating that someone is passing another person on their left side. In the scene from “Captain America: The Winter Soldier”, Steve Rogers uses this phrase as he overtakes Sam Wilson during their run. It’s a common courteous warning to let someone know you’re coming up from behind them to pass.
Sam Wilson repeats the phrase “on your left” not just for etiquette or safety but as a playful way to draw attention to himself and tease Steve Rogers a bit. It becomes a friendly banter between the two characters during their run. Sam’s repeated use of the phrase is a deliberate choice to engage Steve in a light-hearted manner, adding a bit of humor and personality to their interaction. It’s more about camaraderie and playfulness than simply indicating his position during the run.