You spend the whole relationship you have with someone making sure that they always know how replaceable they are. You take extra pains to let them know that they’re not special, and you criticize them constantly. You refuse to make a commitment of any kind. So eventually they just give up and accept that it’s never going to be what they want and need, and they leave you. If you’re genuinely surprised by their decision, I think you need to wake the hell up.