Because every sentient being deserves compassion and care. Because no sentient being should have to suffer. Because their unique experience, thoughts, perspective, and outlook enriches us all. Some of humanity's greatest minds, scientists, advancements, art, and philosophy, have come from people who have lived in society's margins.
If this is difficult to understand, you can try a few things.
First, try putting yourself in this person's shoes. Do you feel like you should have to justify to someone why you should not have to suffer? Does it feel good to be made to feel that way? Do you deserve to feel bad? The answer to all of these is, "no". It is the birthright of every person to have the best life possible. Full stop.
If it's difficult to put yourself in someone else's shoes, you can try thinking of yourself. People who are hurt and have had bad things happen to them often can't help other people. They can't contribute to humanity to the best of their ability. Sometimes, people who are hurt, reflect that pain outward and hurt other people. All things considered, in the statistical soup of the world, if there are fewer people who are hurting, you're more likely to have positive experiences. In Buddhism we call that karma.
“I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops.” - Stephen Gould
Hey. Your path through life so far has been really hard, and that's not fair. No, this life isn't fair. This society isn't fair. This society is quite sick actually. You didn't and don't deserve to be treated this way. You have intrinsic worth and value that you don't have to do anything to earn. I don't know you, but I actually do care about you. What I can do for you is limited, but that's why I'm responding.
I want you to know that even though no one has been through exactly what you've been through, there are people, including myself, that have been falling down a different side of the mountain along side you. I'm a bit older than you, but some of my lowest lows were in my late 20s to mid 30s. I worked very hard to try and tear out the rot in my life and rebuild something that wasn't daily agony, and while my life isn't perfect, it's completely different than it used to be.
I want to pass on some things that helped me in case they might help you. These are just suggestions, and if they don't apply to your exact situation, I'm sorry if any of them cause offense.
Look into CPTSD and the resources for that. There are unfortunately a lot of people out there who have been through these kinds of situations. There are communities who actually get what you're saying because they've lived through it too. They won't repeat things like "It gets better" over and over at you. They'll say things like "Dude, I went through the same thing", "I'm going through this right now", or "It's really hard and I still struggle with it". There are some things that are so incredibly mind-breaking that humans apparently cannot get over. At best we can coexist with what has happened. These are your people.
If it helps, as a temporary defense mechanism to get you through, develop a detached, amused, incredulous, outlook. Like "Wow this is all a horror show, let's see what ridiculous thing happens next." Amuse yourself with the surreal and unbelievable cruel nature of what you and other people are made to experience. But please don't make this a permanent state of affairs since it's unhealthy. This is a tourniquet until you can find actual solutions.
Life gets better by inches and then by miles. This is ridiculous sounding, but it was my experience, and I've seen this pattern repeated so many times when I come across people sharing their life stories as you have here, but they've done this and are on the other side. When you're at the bottom, start from the bottom. Start building basic habits that will form a good foundation for your life. Think about it like rolling a snowball down a hill. For example, if you don't make your bed, just start doing that every day until it becomes automatic. Then do the same thing for another basic thing like brushing your teeth. One day you'll wake up and realize you're taking care of yourself automatically and have the headspace to do bigger things. Here's a site that I still find ridiculous in that it's sometimes necessary but still somehow so helpful: https://philome.la/jace_harr/you-feel-like-shit-an-interactive-self-care-guide/play/index.html
It is much more difficult to take care of your mind if you're not taking care of your body. As part of rolling that snowball, sneak in some things like "I'm going to take a walk every morning."
Accept that some things don't get better, but don't let it define you. I still don't have any friends, and I'm starting to think I never will. I too have an incredibly hard time trusting anyone because of what I've been through. But that's not my entire life, and when I can I try and work on that a little.
I've experienced religious trauma, so it's always hard for me to recommend this, but Buddhism has helped me immensely. It's not dogmatic, you can take the philosophical bits and leave the religious trappings on the floor, and you can practice by yourself. I suggest it because it's a religion/philosophy that starts with your premise (in fact we call it "The Four Noble Truths"):
If that sounds useful, here are some low-effort starting points: http://www.buddhanet.net/e-learning/5minbud.htm https://secularbuddhism.org/starting-out/
You were brave and strong to share your story here. Keep reaching out to people. Unfortunately our society is not built to make it easy for others to care about one another, but deep down, humans really do. I'm not just trying to make you feel better when I say I, a complete stranger, care about you. And I'm also not lying when I say there's very little I can do about it. It's not you, or me, it's the machine we find ourselves in. Still, I do what I can.
Hang in there, and meet yourself where you're at. Some things don't get better themselves, but we can minimize their impact on our overall life.