Just like there is day night and seasons , the world and world events go through cycles
This is true both for individual masses and environmental events.
Those born in early part of 1900 witnessed 2 major world wars. The cry and pain of the victims in Europe reached many sensitive people in USA ! Those sensitive enough became depressed and sad.
The lesson is our thoughts and feelings can be strong enough to be broadcast to family friends and even strangers. It all depends on the strength of the broadcast
These cycles will continue till we learn not to be influenced by then but merely learn to feel the source and have empathy
This is the bare skeleton view. Of course there is more in detail
Bhavin