Let's see, just as an example, on The Dark Knight Returns it's implied that he quit because of the death of Jason Todd. That's a good enough reason to quit, someone that was under his care died, and he felt guilty about it. On Batman Beyond, in the first episode "Rebirth Part I" the reason was physical, he was unable to keep going due to old age. Now let's analyze the background of the Nolan Bat-Films. Bruce does not want to be Batman forever. The ambitions of Bruce are aimed a little higher, a little bigger. On Gotham things are different, it's a unique case in which corruption and crime runs rampant on the city, and the police can't do a thing to stop it, in fact it's part of the problem. The system is broken. A fascinating aspect is that Batman is born out of a need. This is taking directly from Frank Miller's Year One. If the police and the government did their job as they should, there would be no need for Batman.
So Bruce sets his aims on eradicating the root of the problem. It's not about common muggins and robberies. It's the root that causes this problem. In Begins is reflected in the form of Joe Chill. He is the byproduct of the actions of the organized crime and the corruption has done to the city. Creating poverty and hunger. This is also taking from a comic book, The Dark Knight Returns in the first part of the book, where Bruce comes to an understanding of Joe Chills actions and the murder of his parents. At the same time, he wants to inspire people, to shake them out of apathy, and become a symbol for Gotham. Restore the damage, taking Gotham from the hands of crime to their people. Restoring the police, as channeled by his alliance with Jim Gordon, and moving the powerful people into action.
This works in the form of Harvey Dent. This is a man that idealizes Bruce's ambitions, a person with a face that can do things right without resorting to Batman's tactics. He sees that he is almost accomplished what he has set up to do. At the beginning of The Dark Knight we see that things have improved, criminals are scared on the streets, the mob is cornered and people have hope. We also see the unexpected consequences of Bruce's actions too. At the same time Harvey Dent rose, The Joker made his move. We see good people like the Citizens for Batman trying to do good things in the wrong way too. On the other hand, Bruce has this inherent desire for being happy, in his mind to overcome pain and move one, idealized in Rachel Dawes. This is brought upon various interpretations of Batman. See Mask of the Phantasm, where Bruce considers giving up being Batman by the return of Andrea Beaumont.
And all of this fails. The Joker does his act: kills Rachel, breaks Harvey and attempts to gain the upper hand on Gotham's soul. All of Harvey's good deeds could go into the drain if the truth of Two-Face came up to the public. Everything Bruce has done would be lost, people would lose hope. So they cover it up. It is a lie, but Bruce takes on the weight of it, taking the blame, becoming the villian, being hunted.
So at the beginning of Rises, we see a shattered man. He cannot continue being Batman. There is no need. The Dent Act sweep off the remains of the organized crime. If he would make an apparition as Batman for a minor thing, he would be hunted, bringing the police after him, and giving Gordon more pain to deal. (And this happens on Rises) There is no reason powerful enough to go on. As Bruce Wayne he has also lost so many. His project for giving the people of Gotham free clean energy has gone awry. Something that could potentially improve the quality of life of the Gothamites, and namely something he could do as a Wayne, has deemed to be potentially dangerous and is shut down, lossing a lot of money of his company in the way. He only awaits for death.
When Bane and the League of Shadows makes it's apparition, Bruce is mentally and physically broken. He needs to overcome pain and gain a new found respect for his own life. The film takes other points, like Blake being an embodiment for what Bruce idealized on Harvey Dent, albeit in a different manner. Inspiring the people to defend themselves, represented by Gotham's police. Changing Selina's views on the world. And finally, after there is no more to give, he has to make a choice.
So the movie shifts its focus. It's not about saving Gotham, we kinda know that Batman will save it. It's about Bruce saving himself. And because of the journey of this man (as stated somewhat briefly before), we want him to succeed.
Also worth nothing, we do not see Bruce being "happy go-lucky", we see a mature Bruce giving life another chance, a beginning of a new journey.
In my book, he earned this in spades.