Because the pain inside you, strangely, wants to express the confirmation of your inner belief: that you are alone. It somehow feels validated in that process.
And to respond to people who reach out is to open up the possibility, again, of real rejection and pain. But you also, inadvertently, block out the possibility of the love that will cure the sting of feeling alone.
You have to make some choices. Take some actions to change it.
It's not your fault. People hurt us so bad that we put up a wall to prevent further hurt. Including people who means us well.my family hurt me so bad that I don't trust anyone and I don't allow anyone to get close to me or attach. It's just your natural instinct. Work on it little by little.
I don't know either, but I sure know that feeling. I have no desire to get up ( or fix it ) anymore. And when I get home, I just want to be alone. I don't know if you feel the same way, but it's like I give up. My motivational saying is " I just don't care ".
I maybe wrong but reading between the lines l get the impresion that you have been hurt in the past, pushing people away is a defence machanism, building barriers to stop people getting close to you & hurting you again. Its not selfish its just the way you deal with it.