I'm not sure why people get defensive, but my guess it at least has a little to do with that most people recognize that at the very least animals had to directly die for the food (they would be uncomfortable with doing that themselves, or seeing it up close), and that there is a growing awareness of the many negative effects of animal products (environment, over antibiotic usage, poor working conditions, ...). They suddenly feel the need to defend their "decision," when really most people do not really take an active role in deciding that they will eat animal products. (This is by design by the industry, romanticizing animal agriculture and hiding the reality of how that food is produced.)