The first time you see them, is the hardest. You are flooded with emotions that you don't know how to deal with. You've crossed a thresh hold from being together, and being each others "everything" to being almost strangers.
The second time is hard, but not as hard as the first. You recognize them, how different they look but not physically. In some ways you can see them happier, full of life. In others, they can be sulky, sadder. But either way, you convince yourself this is for the best. Whether you ended it or he did, you convince yourself it is for the better, because you will both be happier one day.
The third time is easier. You are past the shock of him, and you have seen how they have changed. Now, you look at the full picture. You see they're better, you see they've moved on, you see the truth in whats happened.
The fourth time, you just don't see him at all. Yes, you physically see him, but it doesn't register. Its like you're an empty vessel just standing, just staring at this guy you once loved walk past like a stranger. Yes, before that day you would've thought that would've been impossible to feel that way, but it isn't.