When the world was finished being created, Jarilo (as the god of love at that time) gave humans and other gods the ability to love, thinking they would love him as the one that gave them this ability. They of course did not, loving others as well, which he couldn't stand. For his fury, he was banished into the depths of hell.
After a while, Jarilo rose from the depths he's been imprisoned in for millennia, only to see humans loving each other like when he left. So he tried to murder them all.
When all other methods failed, the humans of that era tried the only true way to defeat a god of lust and hatred — through love and tolerance. With tolerance and love there were no true differences between them because they were all truly the same, realizing they were separated by their senseless anger towards each other for too long. When they realized this, they all rose as one entity, making Jarilo back off into his hell once again.
In modern times, the Foundation finds the location of Jarilo's pocket dimension in which he hibernated — there, through their desperation (which is, well, a negative emotion) to contain him they activated him again, allowing him to feed off of it and grow. Jarilo almost broke out, ready to destroy humanity; what he didn't expect however was that he broke out on the worst day possible — 14/02, during which a love parade celebrating Valentine's Day, tolerance, love, and romance visited the very place he rose from by accident.
Because after all, through tolerance and acceptance, together, we can even beat gods.
