What are the key factors that can destroy love in a romantic relationship, and how can you spot them early?

Could my girlfriend's accusations of cheating be a sign of her own infidelity, and how can I find out without making things worse?
Will Greta Thunberg ever just go back to finish school and get a real job or has her narcissism gotten so large that we are doomed to her telling us what to think for decades to come?