I too have received comments that I seem calm, though I was experiencing amazing inner stress. While I'll readily rant over daily annoyances, I do not like to emote *real* stress -- feels too much like spirally out-of-control, but this might be an INTP thing.
However, one main reason I am calm, even bubbly, in general is -- as Sonny outlined so well -- due to several prime assumptions: Everyone has their own opinions, and mine is no more important to others than theirs may be to me, therefore win-win or even compromise is best. When people are unpleasant, it tend to stem from the human foibles & anxieties we all have, therefore compassion & empathy is best. Life is full of the unexpected and uncontrollable, therefore flexibility is best. I rarely place expectations on people or events, therefore am rarely disappointed and always happily surprised by the outstanding and good. Tend not to worry about things out of my control. Tend to avoid dramatic situations /people and deflect /deflate conflict wherever it arises.
I like being intensely alive, but mainly through good food/ music/ exotic travels/ hobbies/ learning, while the fundamental of my life is kept very stable: relationships, job, finances, health.
At its best, E9 perspective verges on the zen. Expect much and be haunted by pain. Expect little, and a mote of dust can bring joy.