Can obstacle training make a horse more reactive?

Obstacle training done poorly can absolutely make a horse more reactive and more fearful of novel objects, and this outcome is more common than trainers and owners typically acknowledge because it develops gradually rather than appearing suddenly as an obvious training failure. When obstacles are introduced with too much pressure, too quickly, in ways the horse cannot process and respond to successfully, the training teaches the horse that unusual objects reliably predict aversive experiences — pressure, confusion, forced proximity to frightening things — which builds a generalized wariness about novelty rather than confidence around it. A horse that has been forced through tarps, rushed at flags, pulled over bridges against its will, or punished for showing anxiety at obstacles has not learned that obstacles are manageable; it has learned that obstacles are associated with the handler being demanding and the situation being stressful. That association produces a horse that is more guarded around unusual objects, more reactive to environmental changes, and more suspicious of the handler near anything unfamiliar — the opposite of what obstacle training should produce. Good obstacle training done correctly, with progressive introduction and genuine reward for curiosity and try, should leave the horse more confident and less reactive to novel objects generally because it builds a history of successful encounters with unusual things. Each obstacle successfully processed at the horse's pace adds to a reservoir of positive experience with novelty that makes the next new thing less alarming by default. The difference between training that builds confidence and training that builds reactivity is entirely in the approach: rate of introduction, level of pressure applied, timing of release, and whether the horse's emotional state is read and respected throughout the process.