Cohen died unexpectedly in 1955 one week after having visited Zariski in Cambridge, apparently from suicide. Many years later Zariski said of his death:[1]
Many things are necessary to make a good scientist, a creative man, and left on his own Cohen found himself unproductive. Highly critical of himself and others, he believed that nothing he ever wrote was as good as his thesis. He became increasingly involved with abstract algebra until he found himself at a certain point without ground under his feet. He became disappointed in his work, and finally, fatally, in his own ability.
The Mathematical Association of America (1955). "Report of the Treasurer for the Year 1954". The American Mathematical Monthly. 62 (4): 296. doi:10.1080/00029890.1955.11988629.