Poor people are trained to hide their weaknesses. In low income areas theft, robbery, assault, and murder are common. In order to stay safe one must never show weakness. Rich people have more freedom to be themselves without fearing for their life. As a result, poor people are strongly discouraged from learning vulnerability while rich people have the opportunities to. Because vulnerability is key to how you make friends (How to Make Friends), poor people have a harder time making close friends than rich people do.
Read more about articles in the Rich vs. Poor Series here.