For me, the reason is metabolic, however there are very many reasons to at least eat less meat and dairy products :
George Monbiot writes in "The Pleasures of the Flesh" :
While 100m tonnes of food will be diverted this year to feed cars, 760m tonnes will be snatched from the mouths of humans to feed animals. This could cover the global food deficit 14 times. If you care about hunger, eat less meat.
Monbiot is an interesting writer.
As a designer, I worked in sustainable design for many years, however I still had some cherished assumptions broken, reading his book "Heat" recently. I highly recommend it.