One of the reason there is so much sinas chinam is because it is .... free [בחינם]. Jews like a bargain and even more when it is free. [That also explains why we are so enamored with the idea of "free choice"].
All seriousness aside, there are other reasons why we are plagued by this horrible illness of the spirit. One is that we don't take the trouble to see where other people are coming from, to understand their feelings, experiences, upbringing, pressures, stresses etc. etc. that make them behave as they do. If we did - we would feel much less hostility.
Let me give an example: Many people didn't like President Obama. He was perceived as not being a friend of Israel and instead being a friend and supporter of the Arabs. This was probably true. But let us understand where it came from [besides his middle name being Hussein]. He once said that he really respects all that Israel has accomplished but they have to understand that the country belongs AS MUCH to the "Palestinians" as it does to the Israelis.
That explains a WHOLE LOT. Now, I think that such an opinion is INSANE. Like, who died and gave them this country? Not only was it given to us by G-d [in the same Tanach that Obama believes in] but we captured it fair and square in 1948 and the Jewish State was even recognized by the United States in 1948 and subsequently by the United Nations. An Israeli wrote a 120 page book on the history of the Palestinian people from ancient times until today which contains a quote attributed to the Seinfeld character George Costanza: "Just remember, it’s not a lie if you believe it". The rest of the book is basically blank pages for there IS NO Palestinian people. It is a fiction of the 20th century. [The book was pulled by Amazon.... I guess that they can't take the truth]. But Obama's narrative is that this country belongs equally to us and the Arabs. Ahhhhh. So of course we disagree [so does Hashem] but at least we understand where he is coming from.
So we can use him as a model in our interpersonal relationships. We don't have to agree - but we should at least try to understand. It takes the sting out of many people's bites. Most people are lacking love. If they had received more love and acceptance they would act in more loving and accepting ways. Others are under great stress due to financial strain, health issues, marital discord, problems with children etc. etc. And the list goes on and on. People need a LOT more understanding and a LOT LESS criticism.