i guess one of my favorite things about kutner (so, like, why he is my fave character) is his earnest desire to believe that people can change, that people can be that nice and that patient and that honest, that people naturally suck but they try to be good (that that's what most people do).
kutner has experienced firsthand how awful people and the world can be. he indirectly admits that he doesn't believe in a just universe and that he doesn't truly trust anyone (5x07 the itch), he states that the human condition is superficial (5x10 let them eat cake). he is very well aware that people can and do lie (multiple episodes), he is more cynical and negative than he lets on, he is way more reluctant to trust people than people think etc. he never truly opens up to anyone.
he believes, or at least he sincerely wants to believe that people can change: he tells taub that you can't have your eyes opened to something and not act on it (4x12 don't ever change); he is one of the people that believes that the hassidic jewish woman was genuine about converting even if he doesn't quite understand her, he never once says that this change was a symptom and openly disagrees with house on this. in 4x13 no more mr nice guy, he starts the episode believing that the patient's niceness is not a symptom, he is so sure of it. he tells foreman that "people suck, but they try to be nice, to be good", that that is what (most) people do. he falters on this belief, but ends the episode being certain that the guy is truly that nice, and that he won't change once he's cured. he encourages the soap opera actor in living the dream to find the inspiring thing he wants to do and to go do it.
in 5x13 big baby, with a patient plot a tad similar to no more mr nice guy, kutner meets a patient who is a special ed teacher, who is nice and patient with all her kids. kutner, once again, believes that the traits that make her so good with all her kids are not a symptom. house... believes the opposite. kutner sticks to his guns, and while he is not right about the diagnosis, neither is house: sure, the patient technically had a condition that made her very patient, but she is just as nice with her students after she's cured. kutner is right about believing in people like half the time.
and here's the thing: I think part of why kutner thinks this is that he also wants it to be true about himself.
because kutner was a bully when he was younger (5x11 joy to the world), because kutner's not always nice: he can be kind of rude and has a bit of a short-fuse, because he can get frustrated easily and can be insensitive. but because I think kutner wants to be. and by that I mean, he wants to be a good person (even if I don't think he always sees himself as one), he wants to be nice and he wants to be friends with everyone, he feels a lot of guilt and shame about who he used to be and hates his negative traits (which is why he gets angry at the bullies in joy to the world, he is reminded of his past actions), and so he kind of... "fakes it till he makes it". which is why I think the way kutner acts may appear to be him... wanting to be a hero, wanting to be seen as a hero by other people (see also: him stating that he works as a doctor because he wants to help people), why it may appear a bit like showmanship at times.
so, kutner doesn't really see himself as much of a good person, but he wants to be. he wants to do good in the world. he's changed, maybe (although I think life's messy and even when kutner was a kid and a bully, he still had his characteristic compassion and kindness etc), he tries to do better and to be nice, and so he believes that that's what people do, that that is what people should do (although, he is aware that there are exceptions to every rule). he wants to think that there's good in mostly everyone, because he wants it to be true about himself too. he wants to trust people but struggles with it, whenever he tries to trust people he does seem to be reluctant to fully do so. he's been through so much pain, he's seen the world for all its awfulness... and yet, he really tries to see it for what it could be. because if the world can be better, then so can he.
kutner tries to be a kind person, tries to do good in the world and to trust people (reluctantly, but he tries). and so, he thinks that trying to do better is what people do, because it's what kutner does. because he wants it to be true about the world, and about himself too