Why is everyone getting on Araragi's case for helping everyone? I could sort of deal with it in the Suruga Monkey when Araragi genuinely thought that playing the martyr was the only way to resolve the curse and Oshino just decided to be a dick for no discernible reason and didn't tell him about his alternative measure, but Nadeko Snake just irritated me. Jesus Christ they were just kids! They put the curse on Nadeko to relieve some stress, not because they thought it'd actually kill her! Of course trying to defeat the Snake Constrictor was the right thing to do. Better to try and exorcise it off Araragi when he's right there than to try to find Random A. Guy who probably doesn't even remember cursing Nadeko in the first place. Poetic justice isn't worth much when it kills junior high kids.
It can be Lampshaded in a few ways. The way Kanbaru probably saw it (at the very end of the Nadeko Snake arc) was that the kids should have realized the consequences of their actions before they put a course on her, thus it is karma in action. Also if Araragi was permanently disfigured, Senjougahara would have a field day with those kids.
It's important to realise that, while the below entry explains their lectures to Araragi pretty well, it's not primarily the 'helping everyone' part that people get on his case about. One of the running threads throughout Araragi's Character Arc in the first set of novels is that his usual course of action is "run into something dangerous, push everyone else as far away from him physically and emotionally as possible denying any viable paths of aid or help, isolate himself in order to take all the pain and damage unto himself, and then take whatever action most closely resembles suicide as possible'. Throwing himself onto the grenade is always, literally, his first course of action, and he NEVER thinks about the consequences or how it will effect others. One of the major under-themes of the Bakemonogatari part of the story is the characters screwing up and making their personal situations worse due to poor choices of action, and one of the overt themes for Araragi in particular is recognizing the above-mentioned flaws, continuing the climb out of the VERY dark place he was in prior to Bake (see: Kizumonogatari where he's essentially given up on life), and changing. The climax of the Tsubasa-Cat arc at the end of the Bake novels/tv series is the Sawari-Neko/Black Hanekawa forcing him to realise through her diatribe and faux attempt on his life that (a) he CAN rely on all his companions for help and (b) that sacrificing himself is NEVER the right solution.
Also, yet another reason several of the characters lecture him is for the simple reason that he HAS a significant other, Senjougahara, yet he's constantly playing the knight in shining armor for a wide array of cute girls that keeps growing. And likewise, that he always befriends and begins to hang out, flirt, and generally cross certain boundaries with some of them. Another of Araragi's glaring character flaws, which is a deliberate, serious satire of the usual 'dumb, obtuse harem protagonist', is that he fails to ever realise the effect his actions and attitudes towards the other girls have within his personal sphere. This is why he hurts Hanekawa so badly (although as he himself and Nishioishin point out, this is probably more Tsubasa's fault/flaw than Araragi's).
It doesn't really even need to be lampshaded. It's one of those things that gets lost in the cultural divide between Japan and America, and it shows up in tons of their fictional works. To the Japanese, the idea of "sticking your nose in other people's business" to put it colloquially, especially if you really don't know or aren't connected to those people at all, even if done with the best of intentions, is looked upon as one of a great many respect/manners-breaking faux-pas. Senjougahara accuses Araragi of this when they first meet, and several times afterwards. One of the things she implies often is that its a NEGATIVE quirk of Araragi's that she happens to find charming despite that fact. To put it laconically, the Japanese don't think it's necesarily a good trait, as Americans tend to, to run around wanting to save everyone and doing good deeds against people's wishes for your involvement, ESPECIALLY since this involves things like personal honor, debts being accrued between saviors/victims, and possibly sacrifice on the party of the "defender". This 'concerning yourself with everyone's problems' is not taken to be a good personal trait. Other really good examples of this in anime are xxxHolic's Watanuki Kimihiro (who Araragi seems very likely to be an Expy of and who gets accused of much the same thing constantly by his occult mentor) or Fate/Stay Night's Emiya Shirou.
From what I could tell some of the crap Araragi gets for his constant white knight syndrome is that he way too often insists on plunging in and putting himself directly in the most danger. While he is (theoretically) immortal, he's not completely invulnerable. He could be asking his friends for help more often, since even without their curse powers they're extremely formidable women. (Senjougahara is absolutely frikkin' terrifying just being a "normal" girl.) It often involves him lying to his girlfriend, which she's repeatedly told him really pisses her off. A lot of the people telling him to take the easier paths of just walking away or letting someone else be hurt are often on the more realistic/cynical side of things, Araragi's just insisting on taking the "inspiring anime hero" path, and since on the surface it's not that sort of anime the rest mock him for it.
Does anyone else find it humorous/notable that this series/these novels basically shares almost the exact same description and basic format as CLAMP's xxxHolic, if it were a moe' harem and minus the depth and running/connected plot that accompanies the other property? Probably not a coincidence since Nishioishin is admittedly a fan of CLAMP and xxxHolic (and his professional xxxHolic fanfiction is equally hilarious, since Yuuko and Watanuki are wildly out of character and basically act 100% exactly like, well, Senjougahara and Araragi), but...yeah.
It isn't really a surprise. Nishio even said in an interview that the setting was inspired by xxxHolic, that he liked the more subtle and mystical form of magic that it portrayed
To be fair, I'd say Bakemonogatari and the rest are equally deep, but in their introspectiveness.
How come Araragi freely admits to liking some of the other girls in the show (including his sisters) when he's happily dating Senjoughara (and knows what she'll do if necessary)? I mean, it's funny, but is there an actual reason?
Because they're trying to make him seem like a slightly more realistic person as opposed to the one-dimensional Single-Target Sexuality morons that are usually the lead in a "harem" anime. Your ability to be attracted to other people does not just instantly switch off when you're in a relationship, happy or not, and anyone that says theirs does is lying to either themselves or everyone else.