Peer pressure.

symetry?

He cut his ear off because he drank too much absynthe...He was a tarnished genius destroyed by his own ingenuity.... The universe took him from us, but remember the universe gives us what we want when we want, therefore we being the creators of our own lives. Vincent Van Gogh was the creator and therefore destroyer of his own life... nothing more nothing less.... People look way too much into all this when the answer is simple..... just look at your own life and recognise, whether you like it or not, that you are the creator of your own world... i mean everything in it.... this was maybe not what van gogh understood.....

If you type Van Gogh into the little "Find answers about:" box near the top of the screen you'll find this information very quickly without having to wait for the people on-line now to answer.

A woman made him cut off his ear LOBE (not his whole ear). Also, he used to eat his own paints. Not a pretty picture.

Wikipedia:
Van Gogh cut off the ear lobe on one of his ears during some sort of seizure on December 23, 1888[citation needed]. Mental problems afflicted him, particularly in the last few years of his life. During some of these periods he did not paint, or was not allowed to. There has been much debate over the years as to the source of Van Gogh's mental illness and its effect on his work. Over 150 psychiatrists have attempted to label his illness, and some 30 different diagnoses have been suggested.[65]
Diagnoses which have been put forward include schizophrenia, bipolar disorder, syphilis, poisoning from swallowed paints, temporal lobe epilepsy and acute intermittent porphyria. Any of these could have been the culprit and been aggravated by malnutrition, overwork, insomnia, and a fondness for alcohol, and absinthe in particular.

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Any and/or all of the above reasons, brought on by a disagreement with a prostitute.

The short and sweet answer. . .

It has been speculated that Van Gogh suffered from bi-polar disorder(see http://www.answerbag.com/q_view.php/1380 ). He commited suicide in 1890.

Around 1888, van Gogh suffered a mental breakdown (possibly induced by absinthe) and cut off part of his left ear, which he gave to a startled prostitute friend.

From http://en.wikipedia.org/wiki/Van_Gogh

Didn't know the man but I suspect he was stupid yo...

Actually, van Gogh and Gauguin were friends for a long time, until one day when they were both drunk, they got into a duel. It's believed that Gauguin cut off part of van Gogh's ear during that duel, and the next morning van Gogh didn't remember it, so Gauguin was easily able to pass it off as van Gogh having done it himself out of insanity, hearing voices, or other such legends. This duel didn't take place with swords, it's just a fancy name for a drunken fight back then. van Gogh attacked Gauguin with a razor, and it's probable that Gauguin wrested it from Vincent and cut part of the man's ear off. There's a book somewhere out there all on the evidence of this, but unfortunately I couldn't find it.

Um, perhaps he did it because no matter how hard you try to get people to understand a different perspective, most people just want to hear what they have always heard :)