Peeta does not excel at training, and knows that he has little chance of surviving on his own; if he strikes out as a loner, he is likely to be caught and killed quickly, making it impossible for him to protect Katniss. By allying with the Careers, Peeta is able to keep himself alive and influence their actions and decisions; they are less likely to find and kill Katniss with his hidden interference. When they tree Katniss, he is also the one who suggests that they wait her out instead of trying to kill her outright; this gives her the chance to form a plan for escape. In choosing to hunt with the Careers, Peeta makes a difficult moral choice to condone the systematic murder of other Tributes in order to have the best chance at keeping Katniss alive.
Peeta chose to hunt with the Careers to protect Katniss. He was to lead them away from her. Also, he was able to hunt with them because they were going to use him to look for Katniss, to hunt her down. Basically, he was using the Careers' intention against them.
it explains later on in the book etween a conversation with Katniss and Peeta that he worried for Katniss and her safety and he wanted to try and lead the careers away from her as best he could because he felt he would die anyway but at least help Katniss. the careers on the other hand believe in keeping him around only to be able to find Katniss because they are worried because of the high score she got when impressing the game makers they think she poses as a threat
The careers caught him and became allies with him.
They knew/thought Katniss would be looking for Peeta or the other way around.
The careers were going to wait around for Katniss to show up and they would kill her along with Peeta.Two birds with one stone!
It just didn't turn out that way.=)
They would probably think it was ironic that Peeta and Katniss are dying together since they are "star-crossed lovers" (It is revealed that Peeta actually loves Katniss a lot at the end, though.)
BIGGEST FAN OF HUNGER GAMES!!!
There could have been a number of reasons.
Such as he wanted to protect Katniss by giving them false leads, hints, etc. He may also give the appeal of being the starcross lovers so that the audience would give pity on them and sponsor them, another is that he was only looking out for himself (very unlikely), lastly it may have been a plan set up by him and Haymitch.
to make sure that they dont find Katniss
He chose to hunt with the Careers to do whatever he could to save Katniss, thus increasing the audience appeal (the lovestruck story). It is later revealed that Peeta truly loves Katniss.