Sometimes i feels sadness without any reason

This situation seems quite abnormal but whatever i'm sharing is already happened with me hundred times, also felt by others. I don't know any specific word for this situation. Normally whenever we were sad we mostly have a reason that became us sad. But without any reason, like in normal life you were doing usual things and accidentally feels sadness. This thing sucks and totally changes the mood. I saw a friend crying without any reason on Friday, so that i said him there is nothing to worry. If you have faced this situation you can share your review. Life is too short to live with sadness. 

PEOPLE CHANGE WHEN THEY GROW UP