Rejoicing of the heart is one of the greatest and most important things that helps the servant to worship his Lord and Master as He pleases. However, this rejoicing has causes which the servant must apply himself to seek. Once he manages to employ these means and Allah grants him gladness of heart, he will be on a light from his Lord. Whoever obtains this will have gained the good of this world and the next. This is why Allah, Purity to Him and Most High be He – says:

“أَفَمَن شَرَحَ اللَّهُ صَدْرَهُ لِلْإِسْلَامِ فَهُوَ عَلَىٰ نُور ٍ مِّن رَّبِّهِ”

The meaning of the verse: “Is he whose breast Allah opens to Islam and who thus holds light from His Lord (is he who is not so?)” (Az-Zumar (The Groups): 22).
So, when the servant feels happy, it is a sign that Allah has guided him, while the pang of the heart, which is the cause of the rejection of the prescriptions of Allah and his Messenger - may Allah's peace and peace be upon him - is part of the signs of misfortune. This is why Allah - Purity to Him - also says:

“فَمَن يُرِدِ اللَّهُ أَن يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلْإِسْلَ امِ ۖ وَمَن يُرِدْ أَن يُضِلَّهُ يَجْعَلْ صَدْرَهُ ضَيِّقًا حَرَجً ا كَأَنَّمَا يَصَّعَّدُ فِي السَّمَاءِ ۚ كَذَٰلِكَ يَجْعَلُ اللَّ هُ الرِّجْسَ عَلَى الَّذِينَ لَا يُؤْمِنُونَ” (الأنعام: 125)
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The meaning of the verse: “Whoever Allah wants to guide, he opens his chest to Islam. And whoever He wants to mislead. It makes his heart narrow and embarrassed, as if striving to ascend to heaven.” (Al-An'am (The Cattle): 125).
The type of guidance referred to in this verse is guidance whose meaning is inspiration (of the truth by Allah) and acceptance of the truth (by the servant). Only Allah can grant it, and only he succeeds to whom Allah has granted success.