(Preface) There are many manga where the protagonist/s save lives and are generally very caring people, often putting the needs of their friends before their own but, when it comes down to it, most of these protagonists do these selfless acts because they care very deeply for their friends, not because of any moral obligation. A perfect example is One Piece where the Straw Hats save many towns and even a nation not because its wrong to let people die when you could save them, but because they made friends in those places or because the big bad picked a fight with them. (Don't get me wrong, I love the Straw Hats, I'm just trying to make a point.)

That said, I'm looking for a manga where the protagonist basically goes around saving lives out of the kindness of his heart. The only manga I can think of with a protagonist like this is Trigun but heroes like this are abundant in western comics with characters like Spider-Man and Superman. Preferably, the manga should be seinen/shounen and not have an overly preachy theme.

Thank you.

If Trigun is your ideal example, then there is one protagonist who fits the bill just as well as Vash. The lead male in Immortal Rain is just what you're looking for. If you can get past the slightly rouch art when the series begins, you're in for something amazing. You might recognize some slight similarities to Trigun, primarily because of the concept of people who are apparently "immortal" and the lengths to which others will go to obtain such a thing). Immortal Rain is techinically Jousei, but it's got more chilling creepiness than a lot of shounen/seinen I've seen.

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Well, the only ones I've read that sound a bit similiar to what you've said (from the top of my head, just naming some I'm reading now and like) are Kamui, [m]Katekyo Hitman Reborn[/m] (not really selfless), Natsume Yuujinchou, Ao no Exorcist, and my most favorite out of all that has some of that heroic selfless themes is D.Gray-Man.
But maybe you should include more specific genres? From what you mentioned, I can only assume mangas that have action of some sort and are shounen.. and all of these are labelled shounen, I think (I read mostly shounen mangas because I don't like romance. Kamui does have kinda romance-ish figures though) It would be more helpful, maybe

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"The are some things you can never get back,
no matter how much effort you put in.
And there are some feelings of despair,
that you can never shake off."
-Ciel, Kuroshitsuji

@Alaena Night: Thank you, I took a look at the description of Immortal Rain and indeed, it sounds exactly like what I'm looking for, all that remains is to try it.

@vanillamochi: Natsume Yuujinchou is on my wish list now, thanks. I've read a few chapters of Ao no Exorcist and watched a few episodes of Katekyo Hitman Reborn! and D.Gray-Man but I don't think they are for me, maybe I'll try them again later. Kamui, well, I might try it but it doesn't look like something that I would be interested in.

To give you an idea, I like action, adventure, shounen and seinen but I get tired of shounen cliches after a while and I don't like how pessimistic a lot of seinen stuff is.

Thank you both.

Mushishi should fit, although he does it equally because it's the right thing to do and because he has nothing else he can do.