Made worse by the fact that he was a heavy drinker, as was his wife who died of heart attack at 48. Kate, their daughter, felt that the difference in attendance at the funerals was revealing about the art world (understanding of which must have added to Rothko's depression):

" 'At my father's funeral, people were pouring out of the woodwork,' says Kate. 'But I would have to question why they were there, because only about 10 people came to my mother's. She'd known these people for 25 years, and now it was like she was... ' A footnote? 'Yes. It was disillusioning for me to see the superficiality of the art world, and that has never gone away ..."

http://www.guardian.co.uk/artanddesign/2008/sep/14/art1

Then there was the fight over his legacy, again revealing the sick nature of the art world:

To everyone's amazement, Kate and Christopher Rothko won, and the court issued a crushing verdict. The executors were thrown out for 'improvidence and waste verging on gross negligence'. Reis and Stamos, long-time friends of Mark Rothko, were found to have been in conflict of interest; as executors, they could not negotiate with the Marlborough because the company had both of them on its payroll. All contracts between the Marlborough and the Rothko estate were declared void, and the judge awarded damages of more than $9m against Frank Lloyd, the founder of Marlborough Fine Art, who had laundered art through myriad Liechtenstein holding companies, and the executors."

Ibid

Maybe it's because I'm a Londoner - that I moved to Nice.