Maybe he had Synesthesia, could have driven him insane to hear his paintings.

He was hungry.

THE REAL REASON IS:

One day he decided to paint a picture of himself, so he looked in a mirror and noticed his ear was too big. SO he decided to do something about it. CUT IT OFF!!! but before doing this, he told his lover about his plan, but she disagreed and she said she loved his ears and everything about him, but he still hated his ear... so to make BOTH of them happy, he cut off his ear and gave it to his lover to keep it.

ACCORDING TO THEO VAN GOGH'S (VINCENT'S BROTHER) BIOGRAPHY AND NOTES THEO HAD BEEN HELPING VINCENT FINANCIALLY WHILE HE PAINTED IN AN UPSTAIRS APT ABOVE A TAVERN/BROTHEL. WELL ONE NIGHT THEO RECALLS VINCENT HAD HIS FRIEND GAUGHIN OVER TO LOOK AT HIS WORK AND GAUGHIN SAID HE WAS WASTING HIS TIME. WELL VINCENT WAS SO DEPRESSED TO THINK THAT GAUGHIN THOUGHT HIS ART WAS FUTILE. THAT HE WENT DOWN TO THE TAVERN AND AT THE END OF THE BAR WAS A PROSTITUE NAMED RACHEL WHO WAS SAYING FIFTY QUID FOR AN EAR(NOT CERTAIN THE AMOUNT) SHE WAS REFERRING TO SOMEONE TO LISTEN TO HER. BUT VINCENT IN A FIT OF SELF MUTILATION CUT OFF HIS LOWER EARLOBE(NOT ENTIRE EAR) AND HANDED IT TO HER AND ASKED FOR THE MONEY(SUPPOSEDLY FOR RENT). HARD TO GET BETTER POV THEN HIS BROTHER.

He went to the corn husking store and heard that they were 1/2 off on all ears.

he was angry with him self

I heard he had tinnitus and was trying to get rid of it.

The TV cop show Law and Order: Crimminal Intent put forth the suggestion that Van Gough had cut off his ear because he was suffering from auditory halucinations.
I hnow, this might not agree with the actual facts, but it certainly did work with a pretty good episode!

i think so (but it might be piccasso).
it was a present for his girl friend

Supposedly it had something to do with a prostitute. Although I believe all of the crazy things he did were due in part to the mercury and other chemicals in the paints he used. Mercury is know to make people a little crazy in the head.