Why do I feel the need to pass?
1) Safety -- While ciswomen are always at risk of being attacked by a rapist, robber, etc...being obviously trans* adds the risk of being attacked by transphobes also.
2) Acceptance -- Sadly, if you don't pass most folks will not accept you as the gender you identify as. I would get "Sir" early on even when I was clearly presenting female (dress, makeup, etc...) When I am accepted as a woman, I don't get told I'm not allowed to use the correct bathroom because I make some transphobe who wasn't even in the bathroom "uncomfortable."
3) Dysphoria -- To add to #2, when I am not accepted it makes my dysphoria worse. Being called "Sir" or referred to as "Him" is like a knife in the heart. Hearing "Miss" or "Ma'am" or being referred to as "She" makes me feel so much better. Even more so when someone compliments my appearance - "I love your outfit. You look beautiful."