The Happening/WMG

The movie takes place in the Marvel Universe
Thanos is testing out his Infinity Gauntlet or his Cosmic Cube, and causing the mass suicides to appease his mistress Death. It makes more sense than the plants being responsible.

The movie takes place in the DCU
And the plants beeing sentient and developing the ability to evolve was all a plot by Poison Ivy. It wasnt the power of love that defeated the aliens The Goddamn Batman fixed the problem
 * Actually something like this DID happen once in DC Comics, when the vegetable supervillain Floronic Man, having gained "contact" with the collective consciousness of all plants, convinces them to overload the atmosphere with- oxygen, which would kill all humans (and every other oxygen-breathing lifeform.) He is defeated only when Swamp Thing points out that without those lifeforms, the plants would not be able to breath, either (since their breathing makes the carbon dioxide that the plants need.) A point a elementary school child could have made...
 * So that would mean it was Swamp Thing who saved the Earth. Not the power of love.

It was aliens!!
They were actually the ones doing it, instead of plants as people had assumed. And it wasn't just certain people affected, in the area EVERYONE became controlled, but there were two phases. The strong willed were affected with 'phase one' and killed themselves. But 'phase two' showed no symptoms until finally the aliens become ready to strike. And we don't see when they start attacking their fellow humans with all the knowledge and skill they normally have at the will of the aliens who wish to take over earth.

Elliot was unable to find work as a schoolteacher due to the number of teenagers and children who committed suicide
At the end he’s seeing Jess onto her bus for the first day of school at a time when one would assume he would be getting ready for classes himself.

It was THE BEES!
The bees didn't die off. They just went to their secret headquarters and planned their revenge on the humans.