I studied art history
so did i...
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. However... 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.
Here are a few of the many theories offered...
1. One explanation of Van Gogh's behavior is that he was frustrated by two recent events: the engagement of his brother Theo, to whom he was very attached, and the failure of an attempt to establish a working and living relationship with Paul Gauguin. The aggressive impulses aroused by these frustrations were first directed at Gauguin, but then were turned against himself (Lubin, 1972).
2. A second interpretation is that the self mutilation resulted from a conflict over homosexual impulses aroused by the presence of Gauguin. According to this account, the ear was a phallic symbol (as you mention above, the Dutch slang word for penis, lul, resembled the Dutch word for ear, lel), and the act was a symbolic selfcastration (Lubin, 1972; Westerman Holstijn, 1951).
3. A third explanation is in terms of Oedipal themes. Van Gogh was sharing a house with Gauguin, and Gauguin reported that on the day before the ear mutilation Van Gogh had threatened him with a razor but, under Gauguin's powerful gaze, had then run away.
According to this interpretation, Gauguin represented Van Gogh's hated father and that, failing in his initial threat, Van Gogh "finally gratified his extraordinary resentment and hate for his father by deflecting the hatred on to his own person. In so doing Van Gogh committed, in phantasy, an act of violence on his father with whom he identified himself and at the same time he punished himself for committing the act" (Schnier, 1950, p. 153).
Then "in depositing his symbolic organ at the brothel he also fulfilled his wish to have his mother" (Schnier, 1950, pp. 153154).
4. Another interpretation is that Van Gogh was influenced by bullfights he had seen in Arles. In such events the matador is given the ear of the bull as an award, displays his prize to the crowd, and then gives it to the lady of his choice.
The proponent of this interpretation, J. Olivier (in Lubin, 1972), says: "I am absolutely convinced that Van Gogh was deeply impressed by this practice.... Van Gogh cut off the ear, his own ear, as if he were at the same time the vanquished bull and the victorious matador.
A confusion in the mind of one person between the vanquished and the vanquisher" (Lubin, 1972 p. 158). Then, like the matador, Van Gogh presented the ear to a lady of his choice. (The following explanations, unless otherwise noted, are also from Lubin's  comprehensive analysis.)
5. In the months preceding Van Gogh's selfmutilation, there were 15 articles in the local paper about Jack the Ripper, whomutilated the bodies of prostitutes, sometimes cutting off their ears. "These crimes gave rise to emulators, and Vincent may have been one of them. As a masochist instead of a sadist, however, it is conceivable that he would reverse Jack's act by mutilating himself and bringing the ear to a prostitute" (Lubin, 1972, p. 159).
Interesting stuff, eh :)
There is also the teory that he was out drinking with Gauguin one night and they ended up having a drunken brawl... during the fracas Gauguin atacked VG with a knife, cutting of part of his ear...
VG was so pissed... possibly having danced with the green fairy... that the following day Gauguin was able to pass of the injury as self-inflicted...