What caused Picard to develop an aversion towards Romulans despite serving with them and later becoming their ambassador?

 Jean-Luc Picard's aversion towards Romulans in the Star Trek universe can be attributed to a significant event in his past: the destruction of the USS Enterprise-C and the loss of many lives during the Battle of Narendra III.

During this battle, the Enterprise-C, under the command of Captain Rachel Garrett, responded to a distress call from a Klingon outpost that was under attack by Romulan forces. Despite being vastly outnumbered and outgunned, the Enterprise-C chose to engage the Romulans to defend the outpost, upholding the principles of honor and duty.

This act of sacrifice was deeply significant, as it demonstrated the Federation's commitment to its principles of peace and cooperation, even at great personal cost. However, the Enterprise-C was ultimately destroyed in battle, and many of its crew, including Captain Garrett, perished.

The survivors of the Enterprise-C, including Lieutenant Picard, were captured by Romulan forces and held as prisoners of war. Picard spent weeks in captivity, during which time he experienced firsthand the brutality and hostility of the Romulan military.

This traumatic experience left a lasting impact on Picard, shaping his views and attitudes towards Romulans. Despite later serving with Romulans and even becoming an ambassador to the Romulan Star Empire, Picard's aversion towards them persisted, rooted in the trauma of the Battle of Narendra III and his time as a prisoner of war.

Throughout the Star Trek series and films, Picard grapples with his feelings towards Romulans, often torn between his sense of duty and his personal experiences. His complex relationship with the Romulans adds depth to his character and serves as a recurring theme in his journey as a Starfleet officer and diplomat.