His health started declining in 1973 and he didn't play in any major events in 1974. He died of a heart attack in Helsinki, Finland, at the age of 59, while returning to Estonia from a tournament in Vancouver, which he had won. Over 100,000 attended his state funeral in Tallinn, Estonia.
Reshevsky described why Keres never became world champion: "Well, I believe that Keres failed in this respect because he lacked the killer instinct. He was too mild a person to give his all in order to defeat his opponents. He took everything, including his chess, philosophically. Keres is one of the nicest people that I have ever had the pleasure of meeting. With his friendly and sincere smile, he makes friends easily. He is good natured and kind. Yes, he loves chess, but being a human being is his first consideration. In addition to chess, Keres was interested in tennis, Ping-Pong, swimming, and bridge."
In addition to authoring several books, few people are aware that Keres, who was a great attacking genius and, also, a superb endgame player, published 180 problems and 30 studies.
In the following position, published in Magyar Sakkvilág in 1936, white is a queen up but his K looks doomed on account of the threat of 1...b2+ followed by 2...b8=Q.
The following study won First Prize in the USSR Composers' Contest in 1947. White's task seems impossible, but the win is there. In fact, when I let Komodo 8 analyze the ending, it did not suggest any improvements. It's a fine example of a R and P ending that is worth studying. I would recommend going to the 6-piece Shredder Endgame Database after move 5 and trying out different moves to see how to play the ending.