Everyone is disposable, people just have a hard time thinking like this because of the relationships they have with others. Situations change, people change, sometimes for the better and sometimes for the worse.
Sometimes all you can do is sit back and watch everything you know and love change.
It’s not always bad though because sometimes you realise how much better things are for you now, and neither you or the person you’ve drifted from are willing to deny you of this.
It’s one of those human nature things, where you can’t really explain all of why you left and all of why it seems that your absence is not missed, but almost certainly is.
I really feel like reading some Murakami.