Here's why:
* Katniss chooses Peeta: In the end, Katniss chooses Peeta, ultimately deciding that their shared experiences and their love for each other (even if it's partly a performance) is stronger than her feelings for Gale.
* Gale's feelings: While Gale loves Katniss, he is also angry and hurt by her actions. He believes she is not truly in love with him, and her choice of Peeta further reinforces his feelings of betrayal.
* Real-world implications: Suzanne Collins has stated that she wanted to explore the idea of love and choice in a difficult world, and choosing Peeta over Gale was a symbolic act of defiance against the oppressive Capitol.
While the ending is open to interpretation, the events of the trilogy make it clear that Gale and Katniss do not end up together.