Because they were alone. In fact everyone was alone all the time, but when you got sick you knew it, and that was a lot of what suffering was-knowing.
The quote highlights the pervasive sense of loneliness that can accompany illness. It suggests that while individuals may often feel isolated, being unwell brings this reality to the forefront, amplifying their awareness of solitude. The acknowledgment of being alone becomes a significant aspect of the suffering experienced during sickness.
This experience of solitude during times of hardship emphasizes the emotional struggle of facing one's vulnerabilities. It reflects on how illness can strip away the distractions of daily life, leaving an individual confronting their isolated existence and deepening their suffering in a profound way.