Is it common for people to defend their own positions (that they never felt the need to defend previously) when told of another person's different way of being?

Of all my ‘non-normative’ identities let’s choose three: gender non-conformant, atheist, vegetarian…these are nothing I automatically discuss with people – just usually something I mention at random…the discussion that follows usually centers around why their own gender identity is what it is (with some random studies cited), religious beliefs or meat-eating are acceptable…I always wonder why that’s necessary…it’s not as if I question their positions but they automatically question mine and theirs by way of having to defend themselves…

Do you have this knee-jerk reaction? Does this kind of back and forth (not at all hostile, actually) have a purpose? Is it necessary for people, on a daily basis, to meet others (like myself) who have different ideas so that by opposing their ideas to mine, they can legitimize their life choices? Generally speaking, I figure, adults know what they’re all about and have chosen what they’ve chosen for all the right reasons…yet when I hear them go off on all sorts of tangents that were uncalled for, I am no longer sure that they really know what’s right for them…

Do you ever encounter these situations? I am not looking to be offensive or to start debates with these people (who are usually colleagues) but sometimes I just want to steer the conversation back to where we were (not related to these topics)...I am just not sure where this kind of uncalled for defensiveness comes from…so help me out