Katniss does not find out in Catching fire, but in the first book, when they are in the cave by the river.
Because Peeta loves Katniss
Okay, first of all, Katniss does not know if she loves Peeta or Gale. Her feelings are really mixed up and she has a hard time making choices. Also, SPOILER ALERT!!! (I may be about to spoil the book for you if you continue reading). If Katniss doesn't look like she is madly in love with Peeta--some of the people in the Capitol aren't falling for it--then President Snow threatens to kill Gale and their families. Katniss and Peeta's relationship is confusing for them because Peeta (and even Katniss, sometimes) can't tell when or whether Katniss' feelings for Peeta are real- or just an act for Snow. Catching Fire really explains that Katniss is not that good with this act. Peeta actually loves Katniss, so it bothers him that Katniss has to act. Still, Katniss finds herself falling in love with Peeta. This is made worse by the fact Peeta says to the capitol audience that Katniss is going to have a baby so if Katniss survives, the capitol is going to be wondering 'So...where's the baby then...?'
It affects Katniss because she doesn't know if she has feelings for Peeta. She is confused as to what she thinks of Peeta. Peeta loves her, but she doesn't know if she loves him back.
Because she loves him
It's the last sentence on page 96.
That the girl that he has liked since kindergarten (5 years of age) is Katniss Everdeen.
Yes. It's obvious that Peeta loves Katniss. And by the end of catching fire, Katniss acts as if she's totally in love with him. So inless some thing changes in mockingjay, I'm pretty sure that they are ending up together.(SPOILER)Yes she and peeta end up getting married and having two children
She loves him i guess
Yes he does. In Catching Fire Gale reveals that he loves Katniss.
Katniss doesn't relize it at first but she loves him more than gale.
yes, he grows back to her and they get married
When Haymitch reminds her that all of the tributes are already friends and she only has Peeta. The gamemakers give them both a 12 so they will be targeted first and Katniss does not want Peeta to die, so she decides it is best.