It's summer vacation and Keiichi's parents left him with his five younger sisters to go on an overseas trip. Now with their parents far out of the picture, his younger sisters are aggressively making their moves on him. Of course there's no way he can hold back.

Why do protagonists in anime never die?

I will limit my answer to popular Animes/Manga that I have watchched/read only.
Lots’a SPOILERS ahead! YuYu Hakusho - Yusuke Urameshi saves a kid from a speeding vehicle and gets himself killed in the process.
As a ghost, Yusuke is greeted by an atypical version of the Grim Reaper; a bubbly, cheerful young woman named Botan.
The Spirit World is so amazed how Yusuke, known for his bad attitude, could save a boy, and for these reasons, he was given the chance to live again. Dragon Ball Z - Son Goku, a Saiyan resident of Earth, sacrificed himself by holding his Saiyan brother Raditz in place so that they'd get hit by Piccolo's concentrated Special Beam Cannon which did pierce through the bodies of the brothers, killing them both.
He was later on resurrected by the dragon balls.
Somehow, he dies again when he sacrificed himself, again, when Android Cell self-destructed in King Kai's planet.
Old Kai revives Goku with the cost of his own life. Gantz - Kei Kurono got into an accident in the subway and got killed by the train.
Instantly, he woke up alive in a room with others who have just died like him.
They were revived by a mysterious technology, Gantz, who happens to be a naked bald man inside the weird black sphere in the room, that forced them to fight against aliens.
By killing the aliens, they get to live and achieve points which can be used to revive others who died again. Sankarea - Rea Sanka is a girl with family issues.
She befriended a boy named Chihiro Furuya who is a guy obsessed with or even has a fetish with zombies.
In Chihiro's search of a way to revive his pet cat, he made a "resurrection" potion out of poisonous Hydrangea macrophylla flowers.
In Rea's suicide attempt, she drank the poison but it failed to kill her.
However, following an argument with her father, she fell from a cliff by accident and died.
Just then that the potion worked and resurrected her as a zombie. Yu-Gi-Oh! - Yugi Mutou successfully assembled the Millennium Puzzle; and by doing so, the soul of the Pharaoh from ancient Egypt which was sealed in the puzzle was revived in his body.
Yugi and the Pharaoh are sharing one body, and together with their friends, they fight against their opponents through the card game battle.
In the end Yugi beats the Pharaoh and the Pharaoh’s soul goes to the after-life. Fairy Tail - Natsu Dragneel, a dragon slayer mage of the guild Fairy Tail, happens to have died once when he was a small child.
He died long ago and decades had passed before he was successfully revived.
After his revival, he was taken in by the Fire Dragon Igneel as his adopted son.
He grew into now's Natsu without knowing his past. Naruto - Uzumaki Naruto got Kyubi extracted from him.
He dies and his soul’s consciousness met Hagoromo.
He was later revived with the help of his friends, his dad, and Hagoromo. Claymore - Teresa, a Claymore warrior died when she got beheaded by a rouge Claymore who was out of control.
Teresa's friend, a little girl named Clare, also became a Claymore warrior when she took in her body the flesh of Teresa(Teresa's head).
In the final episodes, Teresa is revived in such a form wherein she is fused with Clare. Deathnote- Yagami Raito had Rem kill L.
L dies but then Near and Melo shows up.
Yagami Raito loses to the combination of N and M.
Ryuk kills Yagami Raito. Code Geass - Lelouch planned for his own death with the help of his best friend.
He dies. School Days - Makoto got stabbed to death and by Sekai, one of his girlfriends.
Then another girlfriend beheaded Makoto’s dead body.
This girl, named Kotonoha, is a yandere who finally lost her mind when she saw Makoto’s dead body.
She took his head then killed Sekai. Devilman - Akira Fuudo got possessed by a demon named Amon.
He was able to regain consciousness and was able to use demon powers even if Amon is dormant.
His best friend turns out to be Satan.
Satan killed Akira/Devilman/Amon in the end. EDIT: i removed Alucard because he was not killed nor revived.
He was able to gather himself in the end and manifested his presence again to serve under Integra Hellsing.