|Self-portrait with Bandaged Ear,|
1889, Vincent van Gogh
1. Frustration over his brother's engagement and his own dismal love life.
2. Self-punishment for his homosexual impulses toward Paul Gauguin.
3. A symbolic punishment for a father he hated.
4. He was influenced by the practice of presenting the bull's ear to the matador.
5. He was seeking his brother's attention.
6. He was seeking to end auditory hallucinations.
That's only about half of them, and some of these are at least plausible. If you think some of them seem far-fetched, the rest are really a stretch.