In the books, Katniss must pretend to love Peeta while inside the arena.
Citizens in the capital are able to send life-sustaining gifts to those competing in the arena. They do so by selecting tributes they like or admire. Thus, by playing on the citizens’ emotions, it’s easier to obtain said gifts. This is why Katniss pretended to fall in love with Peeta.
Now, as the story moves throughout the series Katniss and Peeta are continuously put into situations where their lives depend on each other. In the second book, they both end up back in the arena for the Quarter Quell (75th) Hunger Games.
This forces them to once again pair up and try to survive the games, even though they’ve hardly spoken to one another since the first time going into the arena.
Needing each other to survive + increased heart rate = potential for misattribution of arousal.
Much like real life, when people endure frightful scenarios together, they are more likely to become allies and more trusting.
Think of every movie or show you’ve watched where two strangers are clumped together. As the story progresses, these people begin relating to one another, opening up and even become friends. If the story is between a man and woman, then generally they depict a romantic relationship ensuing. Is that realistic? Perhaps, think of situations in your own life.
With this deduction, it would be logical for Peeta and Katniss to eventually fall madly in love because what would normally be attributed to fear, the increase of heartrate in dangerous situations, is now being expended toward one another.
So, would Peeta and Katniss have fallen in love otherwise? Probably not.
Does that make their love story any less romantic? No, but if you go back and read (or watch the movies), then you might notice small things that will make you go, “Oh! I know what that is.”