Isolation and loneliness are known to be risk factors for suicide. Since the number of suicide cases increased after the COVID-19 pandemic, during which individuals were forced to isolate themselves to prevent infection, this has been re-emphasized as a factor in suicide, and measures to combat isolation and loneliness are promoted. However, few studies have discussed the reasons that isolation and loneliness lead to suicide. Here, we aim to explain the reasons that isolation and loneliness have a significant impact on suicide based on psychology, sociology, and brain science and show that the prevention of isolation and loneliness is directly related to suicide prevention.