In some cases, Apathy descends from dissatisfaction one has faced in his own life. An apathetic would generally be a man whose own desires to be loved have not been sated in the past. Generally such a person doesn't give a damn when he comes across someone in need.
Somehow he convinces himself "When nobody paid heed to me when I needed help, Why should I help anyone?". And as we all believe, We become what we repeatedly do. Thus, voice within him that guides him to help others goes on to become weaker and weaker.
Others, I suppose are too engrossed in fulfilling their own goals and pursuing their own interests that they don't have scope left to look into others' lives and their problems.