Everybody does die, but the animals we farm die on a schedule predetermined from the moment they're bred to be killed. That's not equivalent to the way humans tend to die, in accordance with our intent to have agency and opportunity and so on before fate befalls us. So, humans do die and that's inevitable, but we generally live without a knife on our throats many years before we'd naturally die. Animals on the other hand are farmed to their prime and killed, or used for milk or eggs until their production slows. Then they are killed, again, without any agency or opportunity to live in accordance with their own nature. They live by our desires, die by our desires, and generally suffer immensely along the way. That doesn't sit well with me.
As for our happiness, why does eating meat bring anyone happiness? If it's simple, fleeting pleasure at the cost of a sentient creature's life, it doesn't seem worth it. Pleasure is nice too, but there are better things and better ways to find pleasure than through consumption. Really, the world is filled with countless ways to enjoy myself that don't require the suffering or killing of sentient creatures.
Eating all kinds of foods brings me happiness. They also nourish me better than meat, so meat isn't even necessary. Meat is one small group of foods, and everything else is this massive world of other foods to enjoy. They almost all come at a smaller cost than meat does, financially, environmentally, and psychologically speaking.
That's the gist of it. I get a lot of pleasure from caring for animals. I take time out on my morning walk to pet cats, watch the birds, so on. I dive in the ocean to see what the marine life is doing, record footage to share with people. These things bring me far more joy than a pork chop or a steak. So why choose the steak? If I'm to do that, I have to pretend that it doesn't carry the environmental and external costs of suffering and death. I have to tell myself that I wouldn't rather care for that cow. That's not true of any animal. I'd rather it had a chance to live and thrive, just like I want to.
Some people believe animals don't have the capacity to suffer and that they're lower beings than humans. I see no evidence to support that, other than the very weak "we can't possibly know" or "they aren't as intelligent". These are irrelevant to me. They are more like us than they are different in the vast majority of ways. Hell, we can live with pig lungs in our bodies. An octopus can solve puzzles, as can a raven. Cows mourn when their calfs are taken away. Dogs develop rich relationship with their families, human or otherwise.
You can say no, none of this is evidence that animals are like humans, or that they're sentient. But again, I don't see much evidence suggesting they aren't.
So, the logic is that animals suffering and dying to get on my plate doesn't make me happy. It brings me a lot of sadness. I enjoy other foods just as much, so that then is the logical path forward for me.
I don't criticize others for eating animals. I did for a long time. I raised animals, I hunted them, I fished for them, all of that. It took me almost 40 years to arrive here. If you eat meat and you don't care if animals live or die, suffer or otherwise, that's your call and I'm not for a moment condemning anyone's choices or beliefs.