Most of the vegetarians I know don't do it for strictly rational reasons.

Hi TheRev. I became vegetarian at the age of 20. In my case, the conversion wasn't accompanied by any emotion, and was the result of rational reflection: I just concluded that eating meat caused much unnecessary suffering to other sentient beings. There is of course a selection bias, but most of the vegetarians I know (including many folks at 80,000 Hours, my current employer) have become vegetarians for similar reasons. Here's a relevant comment by Carl Shulman:

I'd say that vegetarians adopt their views for very different reasons The 'spiritual but not religious' types do it because of high empathy/agreeableness, they like animals (empathize with pets, see a slaughterhouse, and you're halfway there). On the other hand, I know ridiculously intelligent rationalists who are driven to it by their intellects.

Highly intelligent people may be aware of the much greater efficiency of plant food vs grain-fed meat: depending on the animal, this saves 50-80% of the inputs. This lowers the cost of food on world markets, and makes it easier for the 3rd world poor to afford, as well as generally improving global allocation of resources.

Very intelligent people are more likely to seek consistency in their ethical views: mammals and birds clearly can suffer pain and are brutalized using modern farming techniques (beaks burned, kept in such tight quarters that they peck each other to death otherwise, etc). Thus, if you're going to oppose killing or torturing human newborns, or severely disabled humans, then logically you should seek to reduce animal suffering.