What was the significance of the character Jeremiah de Saint Amor and the letter he left Juvenal Urbino?

In chapter one, Dr. Juvenal Urbino is summoned to determine the cause of death for his friend Jeremiah de Saint Amor:

The Antillean refugee Jeremiah de Saint-Amour, disabled war veteran, photographer of children, and his most sympathetic opponent in chess, had escaped the torments of memory with the aromatic fumes of gold cyanide.

Because of the smell in the room, the doctor decides not to perform an autopsy. It is easy to see that the man killed himself. The other two men present for the autopsy are soon convinced, and the doctor signs the death certificate. He then realizes that this is one of the first suicides that he has witnessed that is not prompted by love:

And only after . . . did he realize that among the countless suicides he could remember, this was the first with cyanide that had not been caused by the sufferings of love.

The inspector then finds an envelope (addressed to the doctor) that is sealed tightly. The doctor tells the other two men

Nothing in particular . . . his final instructions.

However, there was more in the letter. The doctor finds an address contained in it and eventually visits this address. When he arrives, he meets the dead man's lover and finds out that she knew that he was planning to kill himself:

And then she knew that Jeremiah de Saint-Amour had come to the end of his suffering and that he had only enough life left to write the letter. The doctor could not believe it. "So then you knew!" he exclaimed.

He tells the woman that she should have reported him, but she says she kept his secret out of love for him. She also tells the doctor that when she first started dating Jeremiah, he told her that he planned to kill himself when he was 60 so that he would never be old. Jeremiah wasn't a religious man, so he was not worried about whether or not this was a moral act. His lover views his determination to die without getting old as heroic:

She interpreted this as a heroic determination to struggle without quarter against the ravages of time . . .: he had made the irrevocable decision to take his own life when he was sixty years old. He had turned sixty . . . on the twenty-third of January of that year, and then he had set the date as the night before Pentecost, the most important holiday in the city.

The doctor is 81 years old at this time and is still working and living an active life; the doctor is also much more interested in matters of religion (Catholicism). His friend's unusual decision to kill himself, instead of growing old, provokes the doctor's thought as the novel continues. Though he believes that suicide is a wicked act, he decides that he will attend his friend's funeral.