Why did Vincent van Gogh think he was a burden to his brother?

Theo Van Gogh was Vincent's younger brother. He was an art dealer and one of Vincent's closest friends. Much of what we know about Vincent we know because of the letters he sent to Theo. Along with the emotional support, Theo was of great financial support to Vincent throughout his brief art career. Vincent only sold one of his paintings during his lifetime so Theo provided the money for his livelihood.