I come from a Chinese Malaysian background and spent my early life in the newly separated Singapore. For chinese people middle names are a vital part of our name and are freely told to everyone. In fact when we are more formal, we use our middle names. My name is Yap Voon Yee, or yewenyi in Mandarin. Yap Voon Yee is in Hokkien.
So I was very surprised to find, when I came to Australia as a 5 year old, that Australians are afraid to tell you their middle name. They are embarrassed or even hostile if you ask. They give all sorts of excuses from not wanting to sound like a snob to bank security. It is totally weird.
For the best part of a decade, I looked on in stupefied confusion at this at this weird behaviour. Then one day, I discovered why Australians hate their middle names. I was watching a school friend and her young child. I have seen this behaviour several times since with other people. When the child was naughty, she scolded him by calling out his name in full. So the real reason is: Australians do not like their middle name because they associate it with being a bad person. If they tell you their middle name, they give you power to scold them. Of course, this is an unpalatable thing to do.