A book could be written in answer to this question...in fact I expect scores of books, if not hundreds, already have. What stops people loving one another unconditionally, as most mothers love their newborn child, is fear. Every negative trait in humans is probably based on it.
The greatest mistake is to perceive the world by our five senses only - and nothing else. We deduce that we are all separate, therefore it is a case of 'me against the world,' or, to a lesser degree, 'my family against the world' or an even lesser degree, 'my village against the world.' You could perhaps label these as 'circles of self-interest' with oneself at the centre. The closer the circle to you, the more 'these are my people.' This is why you can read without emotion that 50,000 cars were stolen in, say, Brazil, last year, but get really upset if someone steals YOUR car. It was right in your centre of self-interest.
When those circles eventually become dotted lines, rather than barriers, the whole world will love each other and it'll be a much, much happier place.