Past
As depicted in the episode "White Rabbit" (Lost S1.E05) Jack Shephard has always possessed an impulse to protect others regardless of the risks present. in "White Rabbit," he enters a childhood fight despite overwhelming odds and is beaten up as a result. Subsequently, this impulse is quelled by his father’s crass consultation on what it takes to be a person of importance.
Later in his life, as seen in "Man of Science, Man of Faith" (Lost S2.E01), Jack has become a hardened, fact oriented individual. When he meets Sarah, a patient a car crash victim facing a life of possible paralysis, Jack is at first scientific in his assessment. His father, who has softened with age, is the one who stresses him to project a little hope. Later, a chance meeting with Desmond Hume sets him on a course of self analysis that reawakens his determination to ’save’ Sarah, and he ultimately repairs her spine.
Jack goes on to marry Sarah, but the marriage collapses under the strain of Jack’s disproportionate devotion to his work. Sarah observes, correctly, that Jack’s compulsion to rescue others is ruling his life, and as a result he has isolated everyone around him to the point of alienation.
Present
From the moment Jack’s eyes open in the Lost pilot we see the core of his character kick into overdrive. He dashes from person to person assessing wounds, directing dazed crash survivors to safety, and tending to the injured. People follow his command without question.
As the island’s mysteries begin to unfold, Jack keeps a skeptical mind about anything which does not fall within the realm of reason. When confronted with John Locke’s mystical sense of fatalism, Jack balks. He is determined that the problem that needs to be fixed on the island is the fact that they are there, so his focus is on rescue. Monsters, secret labs, and mysterious ‘others’ aside, Jack’s only motivation is to get his people off the island.
Jack’s relationship with the other survivors has been strained by his single minded approach to island existence. In the episode "Do No Harm" (Lost S1.E20), Jack goes to excessive lengths in an attempt to save Boone, and must be persuaded by the others to let him pass. These extremes are seen in other instances, in Season 3 Jack enters the service of "The Others," with the understanding that he will be let off the island.
In the Lost season 3 Finale, "Through The Looking Glass," Jack finally succeeds with tempered determination by leading his people to the radio tower to make contact with a freighter off shore.
Future
In the episode "Through the Looking Glass" (Lost S3.E23), we get a glimpse into what Jack’s life is like after he does succeed in getting off the island. Surprisingly, we find a Jack consumed by guilt with a desperate mission to return to the island, but we don’t know why.