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I'm going to quote Homer Simpson here: "Just because I don't care, doesn't mean I don't understand". I'm also going to present some reasons behind one part of indifference in our society. I clarify that I do not endorse it, but I can understand it.
People see what's happening around them. Some accept it and some others don't. There are some who are contempt with this situation, and others that despise it.
Given the numerous scandals behind exploitation of the poor, people decide not to help, thinking that it is someone else (a syndicate) behind them being on the streets and asking for money (at least in my country), where the people begging aren't the ones who receive them and try to survive or buy necessities. Heck, some even do it as a job! They've been noted to fight over certain spots. Eg the beggar in front of the local supermarket makes 500€ a week (along with his two companions covering the block), and he's seen riding a car which a person with an everyday job can't afford. The child who is begging me for some money to get something to eat is gathering money for his parents or people who just keep him and tolerate him because he brings money (was Oliver Twist a British production?).
A scenario like this has happened to me quite some times, because sometimes I give money. Here are two examples actually:
a) Child begs for money to eat something. I go and buy him something to eat. He specifically asks it to be put in a box and leaves. Thanks me nonetheless, as he's still a child and (this is my optimist self speaking) understands that I know what's happening, so he sees that what I do is for him and shows gratitude.
Child begs for money to eat something. I go to buy him something to eat. Spits at my shoe, curses me for not giving him money and leaves. This didn't register on an emotional level. I let him go away, but the incident is recorded in my memory for later use.
Most people act by experience. They are not indifferent. Some are mostly hurt themselves and others are skeptical, because they know that anything similar is potentially exploited. Their conclusion is that helping hand-to-hand is neither actually contributing, nor certain of a good deed, as they might have just financed this situation to continue existing.
I currently have no time to analyze the other forms mentioned.