Rajasic happiness is the exact mirror image of sattvic. When the senses meet their favorite objects — a delicious meal, a new purchase, a flattering compliment — the first taste is sweet as nectar. But in the end, that same sweetness turns bitter.
Everyone has tasted this. The first bite of an indulgent dessert is heavenly. The tenth bite brings heaviness. The aftermath brings regret. The pleasure was real — but it carried a hidden cost that only shows up later. That is the nature of rajasic happiness: front-loaded sweetness, back-loaded discomfort.
Krishna is not saying the senses are enemies or that enjoyment is sinful. He is saying: know what kind of happiness you are choosing. If its source is the contact between senses and objects, it will follow this pattern — sweet first, bitter later. That is simply how it works.