Why do people say they like you and care when they really dont?
Why do they say they can vent to you when you cant?
Why do people tell you that your a good person when they don't think that at all?
Why do people say they'll never leave and abandon you yet they always do?
Why do people lie and bullshit time and time again leading you into a false sense of security just to attack and hurt you over and over again?
And why do I constantly believe these people? Why do i let them lie to me? Why do i choose to believe them?
Probably a multifactorial plethora of reasons... Firstly, when it comes to these types of lies, I don't think they necessarily are lies directed to you but rather lies people often like to tell themselves. So, when they say it, they really do believe it themselves. Most people have an unrealistic view of themselves as helpful, accepting and altruistic, without ever questioning what those characteristics actually entails. It makes them feel good to say stuff like that because it validates their own egos. And when they notice what it does, they tend to do what people do best; sticking to their own interests. Those statements should be given way more weight to them, most people wouldn't throw out "I love you" without really meaning it, "I'll never leave" and "You can always talk to me" are for some reason thrown around more losely.
Secondly, empathy very often has an expiration date. Someone can be largely empathic towards someone else at first, but then gradually stop feeling it - often when they notice that there's no immediate reward for their empathy anymore, sort of like how you'd get tired of a really good game if you kept playing it every day only to recieve the same prize over and over again. That thing is something that was confusing for me when I was younger; I've come to learn that my own capacity for empathy is unusually large - I rarely run out of it even if it has happened once or twice thus I have had a hard time understanding when other people has - and I think that may be the case for a lot of people here too since highly empathic people are often suffering. Affective empathy is basically taking on a pain that isn't yours and I think that's an evolutionary disadvantage in the long run, hense why empathy naturally depletes after a while.
Thirdly, human connection has an expiration date. Humans change practically all the time. We absorb new knowledge, have new experiences, gain new interests and suddenly, we find ourselves in places where we notice that we no longer have nothing in common with the very same people who used to feel like our soulmates. Of course there are life long, long-term friendships. But they're quite rare. Rare and quite possibly worth the risk of a few failed ones.
Fourthly, humans are just assholes. The lot of us. And I'm sorry to hear you've been abandoned and attacked. I do think that you believe those people out of hope. Who wouldn't want someone who'd never leave, someone who provide you with support and affection? It's too good of an offer to not believe in it because there's that tiny, little chance that it might be true this time. Don't beat yourself up over it. As I said, those words have more weight to them then people realize.
