To be precise, it was the lobe of his left ear which he put into an
envelope and gave to a brothel wench named Rachel with
these words: "Guard this object carefully." After he tried to
drink a quart of turpentine in his studio, he was sent to the
asylum at Saint-Remy on May 7, 1889. The doctors began to
treat him with hydrotherapy for acute mania and epilepsy. A
precise diagnosis of Van Gogh's illness is still unavailable,
despite hundreds of conjectures. We do know a few facts:
Van Gogh suffered from syphilis contracted from prostitutes
off the docks at Antwerp; there was also a history of mental
illness in his family. Some physicians now believe Van Gogh
may have had a congenital brain lesion that was aggravated by
absinthe.

In December the two artists visited Montpellier and viewed works by Courbet and Delacroix in the Museé Fabre. However, their relationship was deteriorating badly. They quarrelled fiercely about art. Van Gogh felt an increasing fear that Gauguin was going to desert him, and what he described as a situation of "excessive tension" reached a crisis point on December 23, 1888, when Van Gogh stalked Gauguin with a razor and then cut off the lower part of his own left ear, which he wrapped in newspaper and gave to a prostitute named Rachel in the local brothel, asking her to "keep this object carefully."[59] Gauguin left Arles and did not see Van Gogh again. Van Gogh was hospitalised and in a critical state for a few days. He was immediately visited by Theo (whom Gauguin had notified), as well as Madame Ginoux and frequently by Roulin.

he was afraid Paul Gaugin was going to desert him and their partnership...

An alternative theory about this act goes something like this:

In an act of desperation in regard to Paul Gauguin's imminent departure, van Gogh cut off part of his ear: Gauguin and van Gogh were lovers.

He had an ear disorder where he would hear ringing in his ear or he had mulitple personality disorder.

Some research is saying he may have been bipolar.

I believe that the devils brew 'Absinthe" had something to do with it. That, combined with his fragile state of mind were probable factors

He was always mentally unstable. He'd just had a falling out with his friend and fellow painter Gauguin, so he cut his ear off, wrapped it in a handkerchief and gave it to a prostitute down the road.

well actualy, he wanted to make a gift for his wife i think, and well he was a bit crazy...well kinda XD he was in the middle of a deep depression, and back then that = crazy!

According to the movie "Akira Kurosawa's Dreams", he was painting a self portrait and couldn't get the ear to come out right, so he cut it off...

You've OBVIOUSLY never met his wife.