1) Political - The ideology/party spreading the doomerism promotes that they're the ones that can "fix it", if only you vote for them.
2) Adversarial countries, primarily China - Using the internet to convince members of western civilization to hate their civilization, hate their economic system, hate their history, hate their compatriots, adopt a no motivation defeatist attitude, etc., etc., and they can get our population to destroy itself from within without having to go to war with us.
Reject the doomerism, reject the negativity. Build. Invent. Create. Do your part to contribute to your civilization, to contribute to humanity, to move us forwards.