During the interview in the Capitol, Peeta decides to declare his love for Katniss because it becomes an act of defiance against the oppressive regime of Panem. He understands that openly declaring his love will have significant consequences, but he is willing to take that risk to protect Katniss and show the people of Panem that true love and authenticity still exist.
Peeta's genuine feelings for Katniss also motivate him to make this declaration. Despite the pressure and manipulation from the Capitol, Peeta remains honest about his emotions. He recognizes that Katniss is the person he truly cares for and wants to spend his life with, even if it means risking his own safety.
Furthermore, Peeta's declaration serves as an act of rebellion. By publicly expressing his love for Katniss, Peeta challenges the Capitol's control over their lives and choices. He wants Katniss to know that he will always stand by her and support her, regardless of the consequences they might face.
Overall, Peeta's decision to declare his love for Katniss during the interview is a combination of his genuine emotions, the need to protect her, and a desire to defy the oppressive system that controls their lives. It is a powerful moment that emphasizes the strength and authenticity of their bond.