Sabin's personality is almost ironic. With the death of his parents, his strong, rebelling yearn for freedom, and the way he blew up on the night of the coin toss, one would expect Sabin to be dark and pessimistic. He hated his life once, after all, and I think it's interesting that despite all that has happened to him, he is actually a really positive person.
One might assume that Sabin had a luxurious upbringing because he was a prince of Figaro. Riches and royalty? Who wouldn't want those things? Sabin didn't, and that makes the opposite true: Sabin had a hard life because he was a prince of Figaro. His parents were obviously political figures, and were targeted by the Empire for their social standing. It's assumed that Sabin's mother died in childbirth, but I would not be surprised if it was because of poisoning. And while it is only suspected that his father died of poisoning, I would say that's definitely the case. I mean, would you put it past Kefka?
Sabin lost his father at the age of 17, making him an orphan at a very young age. Responsibility was thrown his way immediately, as the people in his Kingdom wanted to know who was taking the throne right away, and that disgusted Sabin. It made him rebellious. He wanted to disown everything about the Empire and run away with Edgar to live a life of freedom where they could make their own choices for their lives. He ended up giving up all of his riches and royalty for a simple life as a monk.
Dark shadows continued to be present in his life. He grew close to a man named Duncan who treated him like a son. Then he seemingly lost Duncan at the hands of Vargas. What's more, is he had to constantly be reminded of these dark things in his life by what was going on around him. Seeing Cyan mourning the death of his loved ones who died by the Empire's poisoning couldn't have been easy for him. Seeing Gau lose his father, even if not in the same way, couldn't have been easy for Sabin.
At times, the dark things in his life break Sabin down. He broke down when his father died. He also broke down when he saw Duncan was alive. That's evidence to me that things do get to him. He just attempts to deal with them differently, and that's through humor. Sabin is incredibly sarcastic, and I think that's a defense mechanism for him. If he's laughing, he doesn't have to be crying. But like I said, occasionally it catches up with him.
The great thing about Sabin is that he does bring light to these very dark situations. He's been through a lot, and has come out on top, so whenever someone else is down, or whenever the group is facing something surely disastrous, Sabin makes light of the situation.
I know, I know. We smash Kefka, then deliver peace into the world.
It definitely puts others at ease, and I think that's one of Sabin's greatest strengths and characteristics. Sabin cares about people. When Cyan had to say goodbye to his wife and child on the Phantom Train, Sabin wanted to talk to him and comfort him, but Shadow told him not to. (You have to have Shadow in your party at the time to see this.) He also wanted to comfort Gau when Gau's father rejected him. He doesn't want his friends to go through the sadness that he's gone through in his life. Sabin even cared about Vargas, helping him see that Duncan wanted Vargas as his successor, even after Sabin believed Vargas killed Duncan.
His sense of responsibility falls down to him caring about people, too, I think. Despite his moment of rebellion when he was 17, Sabin came to terms with his responsibility as an heir to Figaro, and did what he thought was the right thing for Figaro after Edgar asked him what would happen to Figaro if they left, and what would their dad think. It can be argued that Sabin kept a close eye on Figaro because he cared about his dad's desires, but I like to think it's because he cares about people, too, and he didn't want to see Figaro fall to the hands of the Empire.
Without Sabin's optimism, I think Final Fantasy VI would be an incredibly sad game. He brings light to the dark situations the characters face, and I think they all need that.