A vegetarian cannot be understood until you know his/her motivation. Even then it is difficult to get it right. I have met 'vegetarians' who eat chicken and fish??
Some say they do not want animals killed on their behalf, then consume eggs and dairy products. But to get eggs you have to have chicks, and half the chicks born are males which do not produce eggs, so at one day old they are gassed, macerated and turned into organic fertiliser. Most of tehhens end up in confined quarters. To get milk a cow has to have a calf every year. If the calf is pedigree then normally the male calves are used for veal (they usually dont make good beef) or pet food. The females are retained for milk production. Sometimes the milk cows are crossed with a beef bull, in that case all the calves male and female go for beef. So to get your pint, a calf has to die.
The Jains try not to kill, more than most. Because of this they wont eat root vegetables because pulling them kills small creatures. Yet I think they eat yogurt (it helps to digest beans).
Some religions forbid certain meats, usually the religion is based in a hot countryand refers to a meat that goes off quickly in hot wether. Sometimes this folk knowledge is passed on as such.. eg in Cornwall it is said you should only eat pork when there is an 'R' in the month. This skips the hottest weather.