Well she actually started that in Catching Fire. How she "loved" both Gale and Peeta but only when they were in danger.
Even Gale and Peeta thought so. They had a chat about Katniss "loving" them. And they agreed she would choose whichever one she felt she needed more.
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"THIS is how you make Nintendo win another generation. Good job, Sony and Microsoft, you're idiots." - Vlado