When you're in a relationship, if it's been good you become entirely fixated on this person being your main source of communication, laughter as well as everything else that goes with it depending on how intimate you get, they're your best friend and your lover. Your life ends up working around your partner without you even realise it, they take up all your free time because you don't much care for giving it to anyone but them because they make you feel so great. Most break-ups come suddenly and abruptly for the one who gets dumped, you end up not having them there to talk to, or to spend time with, the time you spent talking to them now becomes too much for you to know what to do with and you end up just thinking about the time together even if you don't necessarily want them back. Whilst you thought everything was fine, they were slowly developing a plan to break free without letting on to you. It's not easy for them either as although they ended it, they feel guilty for how bad the break-up has made you feel, and they too have too much time on their hands, a void they can't fill. It also can end up making you realise negative things in yourself, worsen a depression you never realised you already had, makes you see how little friends and social life you have besides when you were with that person, although not always the case