Presenting Your Characters’ Backstories

I was recently sitting at home with my wife when it occurred to me that if one of my schoolmates (whom I haven't seen in years) hadn't gone on holiday to Spain in 2002, I would never have got married more than a decade later.... A leads to B which leads to C, all the way up to Z and beyond. This is what happens in real life. In fiction, this needs to be compressed into a backstory. Where did your character come from? Who influenced them as he was growing up and throughout his life up until this point? And most importantly of all: what is that defining moment in their past that made them who they are today?