I stopped eating meat 25 years ago. It was my first year at university and I was studying for a module on ethics. I had an essay to write on utilitarianism and animal rights, and (as sadly I often did then), I'd left it till the last minute to write the essay. I stayed up all night reading and writing. At some point during the night I thought to myself, "I'm convinced by this argument. I don't want to eat animals anymore." So I stopped. Apart from a few blips (when staying in a village in Ghana I ate everything I was given without quibbling; I also went through a sausage sandwich phase around that time to "practise eating meat") I haven't eaten meat since.
There are lots of reasons people reduce their meat consumption, or cut it out altogether. Back then, the argument that convinced me went like this. Utilitarianism in ethics is the idea that the right thing to do is that which produces the greatest happiness for the greatest number. When we intensively farm and then kill animals in order to eat them, we're producing great suffering to sentient beings for a relatively trivial benefit: eating something enjoyable. We can survive perfectly well on vegetarian food, perhaps with some vitamin supplements. Those who know me well may smile at this point, as they know that I eat fish. I've often thought that I'd like to stop eating fish too. There is some logic there: it's the animal welfare implications of intensive farming that really upset me. I have more of an issue with keeping animals in inhumane conditions than I do with killing animals for food. (A vegan once pointed out to me that killing animals isn't exactly humane. He had a point).
I've also become uncomfortably aware in recent years that dairy and egg production can be very inhumane too. Then I read 'There is no Planet B' by Tim Berners-Lee a couple of years ago and was fascinated and horrified to discover that the environmental arguments for veganism - or at least for cutting right back on meat, fish and dairy - are just as compelling as the animal welfare ones. We're facing a global climate emergency which will negatively affect every sentient being on earth. Having steak and chips, or a delicious platter of sushi in my case, is pretty trivial in comparison. I've often toyed with the idea of going completely vegan. So this January I'm going to try it.
Check out https://veganuary.com if you're interested in trying it too.