Defensive? I’d say that I’ve probably quietly felt defensive for years as I went through life trying to live in sync with common values and ethics because I wasn’t out inviting people to come in to our church or singing loudly during the hymns (and this was at the mainstream church). Never mind trying not to roll my eyes at any sermon that started to mention the words of Paul.

If anything, i feel a need to voice my beliefs. I think I'm right, and I want to challenge my beliefs. If I can change a mind or two, bonus. I also come from a 7th day adventist background.

Obligated, No.

But I often engage with theists, and will always engage with science deniers/my faith is greater then facts. (Such levels of stupidity can not go unchallenged.)

Sometimes I will bring them to tears when they discover how void of facts the foundation of their belief structure rests.

but it does depends on the situation. Lately I will just roll my eyes as I am so surrounded by so many delusional superstitionalists.

Yes, sometimes I feel obligated to defend. Then I realize that it is like taking a pacifier away from a "teething" baby.

The only person I have ever felt odd about discussing atheism/agnosticism with is my mother. Actually we have never really talked about it. She still talks about religious stuff when we talk, not trying to change my mind, just local goings on.