Ibn Qayyim mentions:
Nothing can cure the heart that is sick except making it sincere (Ikhlaas) and directing it to singling out only Allah with His Ibaadaat (worship). Shaykh Fawzaan comments: from the signs of a good heart is that the one who possesses it, if he misses out performing the sunan acts, he feels hurt and pain more than the pain he feels whenever he loses his wealth.
The shaykh mentions the example of the sincere believer missing Tahaajud (night prayer) becoming hurt because he knows that he lost something great and this is from the example of the prophet صلى الله عليه و سلم whenever he missed performing Tahaajud in the early hours of the morning, he would make up for it during the day, even during travel he would make up for this great prayer on his riding beast as a cure for his heart from the pain he felt whenever this happened to him.
Shaykh Fawzaan mentions: the believer always connects his heart to Allah Azawajal in order to make him do good deeds until he meets his lord.
[Class notes from Igaathatul Lahfaan of Ibn Qayyim by Abuabdilmaalik Abdulhaqq Ibn Islaam]