Who is at fault here? I'm joining from the left. She/He is joining from the right. She/He was not there when I entered the lane, but then decided to enter when I was there. I'm honestly not sure who is at fault here.

Who is at fault here? I'm joining from the left. She/He is joining from the right. She/He was not there when I entered the lane, but then decided to enter when I was there. I'm honestly not sure who is at fault here.