why does greta thunberg always think she has to lecture us about climate change?

Honestly? I mean if we’re being honest. No one commenting here, and the vast majority of people, have no reasonable, intellectual basis for knowing one way or the other to what extent global warming exists and to what extent it’s caused by man as opposed to natural climate variation. Unless you have an understanding of physics, fluid dynamics and other complex disciples, you’re just parroting whichever narrative you choose to believe.