The diseases of the heart revolve around four things: corruption, weakness (i.e. unable to restrain oneself), forgetfulness (i.e. due to being heedless) and oppression.
Doubt, ignorance, confusion, misguidance, love of deviation and wicked lowly desires (found) in heart returns back to these four affairs (i.e. corruption, weakness, forgetfulness and oppression); so a slave (i.e. a person) pursues the causes of the heart’s sickness until he becomes sick (i.e. his heart becomes sick), then Allaah punishes him by increasing this disease due to him giving preference to the causes of this disease and pursuing them.
[Source: Badaa’i At-Tafseer Al-Jaami Limaa Fassarahu Al-Imaam Ibnu Qayyim’ Vol 1. Pages 96-97]