The poem starts with something ordinary: people look at each other, eyes meeting eyes. That sounds like connection, but the second line takes it back. Between "you and I" the rule doesn't hold, and the only thing the speaker sees is the other person's back. The look never happens because the other person has already turned away.
The final line, "You left me behind," says it plainly. The speaker feels abandoned, and the word "behind" does double duty: it describes where the speaker stands, literally behind someone walking away, and where they are emotionally, stuck while the other person moves on. The title, "Staring," suggests the speaker is still watching, maybe still hoping for a glance back.
The mood is quiet, bare and a little bitter. The short lines and simple words make the hurt feel unvarnished, as if the speaker doesn't have the energy to dress it up. It reads like someone saying the truth once and then going silent.