Animals have a mind that has the ability to think, hence they will feel fear and pain when slaughtered. As a result, eating the flesh of animals is detrimental to a person’s cultivation of compassion. As for plants, they do not have the ability to feel pain or happiness, so the feeling generated from plucking a leaf and killing an animal is after all, different.
Reference: Master Hsing Yun “Problems of Contemporary Society” (U.S.A, Los Angeles, University of the West Seminar)