In The Cellist of Sarajevo, Steven Galloway tells the story of four people who serve as a microcosm of country as it faces a war. The soldiers are encamped in the hills around the city, making the people of Sarejevo east targets for their their hatred and the bullets.
Each character can succumb to the hatred and ugliness of war or fight against it. Your question suggests their happiness rests in their own hands, and that may be true. What is certainly true is that, even in this uncertain setting, they each control their fates as much as anyone can in such uncertain and perilous times.
The title character is the musician who plays the cello. A former member of the Sarajevo Symphonic Orchestra, the musician is moved to do something after watching a horrible event outside his window. While standing in line to get bread, twenty-two innocent people are killed by a mortar explosion. The next day, he sits in the hole left by the shell and plays a tribute those who died--and he will do that for twenty-two days. The musician's playing is his way of controlling his destiny. After he finishes playing his twenty-second musical tribute, he quietly places his bow in the weeds, weeping for what has been lost.
Arrow is a female sniper. Nothing in her past suggests she would be in this position, but here she is. Now it is her job to protect the musician, a job she does not willingly take; however, she does it. The musician is a source of hope for the people in Sarajevo, so it is in the best interest of the enemy to kill him. She does not let that happen, killing the sniper who is also moved by the music. After she sees the musician leave his bow, she does the same with her rifle. It is an act of defiance, but she knows this is what will make her happy, even as it precipitates her death. It was her choice to make, and she chose to control her own destiny.
The men in the hills didn't have to be murderers. The men in the city didn't have to lower themselves to fight their attackers. She didn't have to be filled with hatred. The music demanded that she remember this, that she know to a certainty that the world still held the capacity for goodness. The notes were proof of that.
Kenan is a husband and father who simply wants to provide for his family, and he is having serious trouble doing that due to the war. It is not his nature to be bold and courageous, but his family's need for water overrides his natural tendencies. He even agrees to get water for his cranky and uncooperative neighbor. The trip is treacherous, and it ends, as most things do these days, with a battle. The literal battle is the enemy attacking innocents; the internal conflict is Kenan's anger at being in this situation. He is not going to master his fear--until he hears the sound of the cello. The music gives him new hope, and he is able to conquer his fears, take control, and complete his task.
Kenan has sent his family away, but he feels he must stay and do what he can to save his city. Though his motive is admirable, it is not an easy task. In fact, he, too, finds himself living in fear. When a family friend is killed trying to deliver medicine to someone who needs it, Kenan remembers the music and refuses to let the enemy win. He delivers the medicine and has a new calling as a leader of those who remain behind to fight for their survival.
All four characters are determined to control their own destinies and pursue happiness; to the extent that are able, they do just that.