Among Westerners, vegetarianism is mainly a matter of personal choice. There are Christian vegetarians, Jewish vegetarians, atheist vegetarians, agnostic vegetarians, and secular-humanist vegetarians. Among certain other religious communities, however, vegetarianism is an integral part of the community's religious practices.
Hinduism is one such religion. In general, Hindus do not eat beef or other meat because they consider animals sacred beings, capable of bearing the incarnations of gods. Note, however, that it is important to say "in general," because the question is more complicated than that of a hard-and-fast rule about eating meat or not eating meat.
To see exactly what this means, take a look at Nine Questions About Hinduism. Scroll down to Question #4, "Why do Hindus regard the cow as sacred?" and #6, "Is there a rule about Hindus eating meat?" What you find there might surprise you.
You can find links to these and other sources on religions at a McKinley Yellow Pages 4-star Web site, Facets of Religion. For still more links, try the McKinley Magellan Internet Guide.