Don't Be Sad over misfortune or Too Glad over Success because..

مَآ أَصَابَ مِن مُّصِيبَةٍ۬ فِى ٱلۡأَرۡضِ وَلَا فِىٓ أَنفُسِكُمۡ إِلَّا فِى ڪِتَـٰبٍ۬ مِّن قَبۡلِ أَن نَّبۡرَأَهَآ‌ۚ إِنَّ ذَٲلِكَ عَلَى ٱللَّهِ يَسِيرٌ۬ (٢٢) لِّكَيۡلَا تَأۡسَوۡاْ عَلَىٰ مَا فَاتَكُمۡ وَلَا تَفۡرَحُواْ بِمَآ ءَاتَٮٰڪُمۡ‌ۗ وَٱللَّهُ لَا يُحِبُّ كُلَّ مُخۡتَالٍ۬ فَخُورٍ

No calamity befalls on the earth or in yourselves but is inscribed in the Book of Decrees (Al-Lauh Al-Mahfooz), before We bring it into existence. Verily, that is easy for Allah.
In order that you may not grieve at the things that you fail to get,
nor rejoice over that which has been given to you. And Allah likes not prideful boasters.

(Qur'an 57:22-23)

In this verse, Allah SWT consoles us not to be sad over misfortune as it was already predestined for us. One must be convinced about the fact: 'What has met you would never have missed you and what has missed you would never have met you. Therefore, do not grieve for what you have missed of fortune, because if it was destined for you, you would have achieved it.'

Allah SWT then says 'Do not boast before people about what Allah has favored you with, because it is not you who earned it by your efforts. Rather, all this came your way because Allah destined them for you and provided them for you as provisions. Therefore, do not use what Allah has granted you as a reason to boast and become arrogant with others.'

Ikrimah commented on this by saying, "Everyone of us feels happiness and grief. However, make your joy with gratitude and endure your grief with patience.''