Charlie had much to live for:
1. He wanted very much to protect Claire and Aaron.
2. He wanted to have a family with Claire and Aaron.
3. He wanted to get his career back on track, since the island gave him a second chance at life.
4. He wanted to be a hero, so people would look upon him not as a "one hit wonder," but a real person.
His relationship with Claire was not that unusual. Claire was the damsel in distress after the crash. Who wants to deal with a pregnant woman in shock? But Charlie did - - - instinctively. But initially Charlie felt he did not have the skills to impress and keep her: Jack was the medical savior protecting her baby, while Locke was the hands on guy who could build her shelter and a crib. All Charlie could give Claire was kindness, something that apparently was lacking in her life.
And Claire did not know how to react to Charlie's affection. She was put off by his imposition of himself into her island situation. She did not have the same feelings for him. Some would say her hormones were all out of whack, and the added stress of the Others wanting to take her child made her mad. But even as frustrated as Charlie got, he never gave up. When she was kidnapped, he went into the jungle to confront the Others - - - and he wound up hanged by a tree. Jack had to cut him down - - - which brings us to an island tangent: did Charlie actually "die" in that encounter to be reincarnated as a smoke monster or soul seeking forgiveness of Claire for not being able to protect her like he had promised her?
If Charlie was Charlie 2.0 (soul/smoke monster/reincarnation) that would put a whole different spin on "what" the island was . . . . beyond a metaphysical dimension in time or space but a soul proving ground for redemption.
But after Desmond's purple fail safe moment (which like Charlie's hanging should have killed Desmond, who was found naked in the jungle), Claire seemed to gravitate toward Desmond rather than Charlie. It made Charlie jealous. Then when Desmond told Charlie he could see future visions, including Claire and Aaron leaving the island on a helicopter, Charlie knew he had to make that happen. But when Desmond told him that he also saw Charlie dying - - - they connected the two visions as being a cause and effect. In order to save Claire, Charlie had to die.
There were no "rules" which made that connection true. Charlie's own weaknesses: his low self-esteem, his jealousy, his rejection, his self-pity, all contributed to his suicidal but heroic stance in the control room. In order to radio for rescue, Charlie had to recognize a musical pattern code to unlock the panel. This always seemed to contrived to be true reality. But it made Charlie the "only" person who could figure it out - - - his own supernatural power. But once he got contact established, he found out that Desmond's vision was wrong: it was not Penny's boat coming to save them. It was Widmore's freighter coming to kill them.
So instead of doing anything possible to bring that news back to the island - - - and to protect Claire from the coming harm - - - Charlie decided not to open the control room door. As Desmond pleaded with him to open it, Charlie drowned in what could be considered a senseless death.
The only thing that Charlie's death did was to cause other people, especially Desmond, pain. Dez's flashes were not reality and not true premonitions. Desmond's own personal motivation to get back to Penny clouded his judgment. It cost Charlie his life. It cost Charlie his chance to make things right with Claire.
After Charlie's death, Claire went insane when Aaron was taken away from her. Some believe that Claire may have been killed during that three year period of darkness - - - since she could see "Christian" a smoke monster, she too could have been recreated into one. She did not ask about Charlie at all when she encountered Kate. She did not miss him. Her sole focus was revenge.
So why did Claire rejoin up with Charlie in the after life? There connection was broken on the island when they were not on the best of terms. The sideways was an afterlife plane of existence, but it has the troublesome unreal aspect in which Aaron and Sun's baby have in common:
Why did the island pregnant women give birth to their children in the afterlife if they had already been born in the real world?
Logically, an afterlife birth would mean that Claire and Sun never gave birth in their real worlds. That would mean the island was not in fact real. Their motherhoods were illusions. Their relationships and interactions with other people were merely dreams. The sideways world was the ultimate "do-over."
Charlie got his second chance with Claire, to experience the birth of Aaron. Claire got a loving partner in return. Their reconnection seems to be the most real of the final pairings, as we still have issues with how Jack and Kate wound up with each other while Locke never reconnected with Helen.
