I suppose I should toss mine into the mix, too.
Rory :: When you don't feel safe in your own home, it's natural to seek out safety elsewhere... and if you don't find it, to retreat inward. Rory's Semblance stems from his desire to shut out the world, to feel safe within himself -- and like Rory himself, this Semblance (and the safety it offers) is far from perfect.
Sean :: This one is a case of "which came first, the chicken or the egg?". Sean has always been a bit of a firebrand, wanting to bust out of the "safe" shell of his restrictive home life and leave his mark. His desire to live dangerously, to drink in experience, and to "break free" factors into it a lot -- his personal philosophy sees a lot of similarities between a life well lived and an explosion, viewing both as brief but intense sources of "light" and "heat" that return to dust and ashes in the end. It's hard to say whether he had this perspective before discovering his Semblance, developed it afterwards, or if the entire thing occurred in parallel.
Alec :: Alec's Semblance stems from a youthful belief in strength, to put it simply. Hard life experience, however, has taught him that strength is not enough on its own, and as his own prowess has faded with age so too has his Semblance begun to dim. That said, in moments of dire need it's still there for him -- an old man's act of desperation born from a young man's wish to take on the world.
Cordell :: Ah, Cordell's "treasure sense". This Semblance was born out of hard experience during her early days of piracy, the stress of a raid mingling with the urgency of finding both "the good stuff" and knowing where threats were going to come from. Experience has honed it into a true sixth sense, but at the outset it was a confused glowing garble in her mind (along with an intense headache).
Micah :: When the defining moment of your childhood is being pushed into a campfire and covered in life-changing third degree burns, it leaves marks both physical and mental. In some ways that fire is still burning its way through Micah's soul even decades later, but he's learned to make use of its byproduct. "Where there's smoke", after all...