Because he recognized Katniss from the beginning. While Haymitch and Katniss had to work out their connection, Cinna and Katniss were inseparable from the start. It's apparent that Katniss felt at ease with Cinna and that she cared profoundly for him, as he did for her. But what happened to Cinna after Katniss left for the Reaping is unclear.
It's possible that Cinna followed Katniss out into the world, looking for her and never giving up hope that they would be together again. Or perhaps he didn't want to be a burden to anyone else when so many people were suffering during the Games. Either way, it seems clear that Cinna loved Katniss too much to let her go forever.
In fact, there are several clues in the book series that suggest that Cinna may have been killed off-screen. In Mockingjay, it is suggested that Cinna is dead.
Katniss and Cinna's friendship is loving from the beginning of their time together. He's kind with her, telling her he's sorry she was reaped and that he'll do everything he can to help her focus herself and prepare for the games. Cinna is essentially Katniss's Capitol best friend. They laugh together, they cry together, and they're there for each other when it matters most.
Cinna does have a girlfriend during this time, but they break up before the Games begin. This doesn't change anything between him and Katniss - she still sees him as a friend who cares about her well-being.
In addition to this, Cinna is one of the few people who knows about Katniss's identity as the Mockingjay. She tells him before the start of the Games, in order to get his help finding Peeta. However, she doesn't tell him why she needs to find him, so it's possible she didn't trust him not to tell someone else.
Finally, Cinna is one of the people who goes into the Capitol after it has been destroyed by District 13. He dies along with the other victors in the Capitol bombing.
Overall, Cinna is a good friend to Katniss. They care about each other and want what's best for each other.
Cinna had aided Katniss with her clothing selection in what way? Cinna assisted Katniss in obtaining sponsors for her return to District 12. Katniss needs to be strong and aggressive in order to demonstrate her resolve in winning the games. Thus, Cinna helped her obtain items that would make her look fierce and appealing.
Katniss used her clothes as a form of self-expression. She designed her own outfits and wore what she wanted. Cinna helped her become more comfortable with herself by letting her wear whatever she wanted.
Here are some examples of costumes that were created specifically for Katniss: The first is a bow and arrow costume that she wears during the Reaping ceremony. It consists of an archery shirt, tight pants, and leather boots. The second example is a cowboy costume that she wears when she visits President Snow in his mansion. It includes a blue dress with fringe details and white leather boots with heels.
Katniss also uses her clothes as a form of protest. For example, when she goes back home after being named Hunger Games winner, everyone throws water on her instead of candy. This means that they want her to live rather than die. Also, when she returns for the Victory Tour, everyone is wearing clothes that symbolize their opposition to the Capitol - black for Peeta, red for Finnick, and yellow for Gale.