What are triggers?

I have a question about “triggers” – would you define these as preformed neural pathways that are laid down over time as automatic responses to certain stimuli? An automatic firing of the limbus system? Is a trigger just a more intense version of a belief that is linked to excessive emotional arousal?

Can a trigger ever be positive and lead to excess joy and positive neurotransmitter release, instead of the inflammatory cytokines associated with negative emotion?