Here's why:
* He notices Katniss's feelings for Gale: Peeta is perceptive and observes Katniss's close relationship with Gale. He understands there is a deep bond between them, which likely makes him uncomfortable.
* His feelings for Katniss: Peeta loves Katniss and wants her happiness. He wants to be with her and be the one she chooses. Seeing her with someone else, even platonically, is likely painful.
* His emotional state: Peeta is dealing with trauma and emotional stress from the Games. This can make him more sensitive and prone to jealousy.
* The kiss: While it's not a romantic kiss, it still represents an intimate moment between Katniss and Gale, which is likely to be a trigger for Peeta's insecurities.
Instead of outright anger, Peeta reacts with sadness and withdrawal. He expresses his disappointment and tells Katniss he understands she has feelings for Gale.
It's important to note that Peeta is not a villain in this situation. He is a complex character dealing with his own vulnerabilities. His reaction highlights the emotional turmoil Katniss's actions cause for him, and contributes to the complexity of their relationship.