"Some things I'll never know..."

I don't understand how some people just... don't care about certain things. Things that you always thought meant so insanely much to them and stuff that meant just as much to you. You talk to them about it and they say they'll change and feed you fucking bullshit and lead you into something and then completely go against it. How do people just suddenly stop caring about something? I don't know about those people, but I can't just let something go. I guess sometimes that's a bad thing but especially strong feelings about something; I don't just let that shit go. I can't help but think about it. Even more so if it's a person they stop caring about. How does that happen? It's not like you can erase them from the face of the Earth; They're always going to exist somehow in your life. How can you just block that shit out? It's beyond me. Either you have to be insanely emotionally numb or just hide your emotions really really really well. Either way, how do you block out how that other person feels? People were meant to exist with one another, so how do you just block them out? You can't just live off yourself you have to communicate to... live.I guess I just feel like I'm wasting my time with some people who don't even seem to care about me. I mean, I'm not an outgoing person and I don't usually make plans with other people; I'm extremely introverted but I care about certain people a lot and I'm really tired of giving for nothing.