Why do Kat and Peeta decide to talk on the roof?

If you're talking about the first scene where they talk on the roof in the first novel, Katniss couldn't sleep and needed air, and when she went to the roof, Peeta was already there for the same reasons, so they stayed and talked.

Well I think they talked twice but the one I remember is that Katniss wanted to tell Peeta about how she knew the red headed avox.