When Six decides that the only way to get the Autumn off the planet is for him to stay behind, Legion's reaction. Think about it, Legion is an AI, so he knows exactly what the odds are of Six surviving are. He experiences time differently, so he's going to be spending a very long time wondering how long his friend will live. But more than that? Shepard was the first to reach out to the geth. Tali was the first creator in hundreds of years to speak of peace with them. But Six? For Legion- no, the entire Geth Race- Noble Six was their first friend. And possibly the worst part of all is that the way the scene is written, you can tell that Legion has no idea what's going through his mind, but as much as he claims to be emotionless, you can feel the rage, sorrow, and grief pouring off of the platform. The Geth have lost their first friend.