we can argue about this all we want, but at the end of the day, kutner's suicide wasn't meant to be foreshadowed. that's what all the interviews say: it was meant to come out of nowhere (at least at first glance), there was meant to be a lack of answers or reasons behind it, it was specifically meant to screw with house: the guy who has all the answers doesn't know why kutner, a fellow who has worked with him for 2 years, killed himself. that's why house tries to turn it into a mystery and comes up with the Crack Theory that kutner was murdered: if kutner actually did kill himself, then it must mean that house missed the signs, ergo, it puts his ability to solve puzzles into question(the only thing that matters to house, his brain); ergo, it means he is guilty for not noticing the signs and then doing something about it; it's also a distraction from his grief, making kutner's suicide into a puzzle. (an article mentioning some of these things)
superficially, kutner is the least likely character to kill himself. thirteen was kind of passively sucidal for almost the first half of season 5; taub, well, is heavily implied to have attempted suicide. kutner, next to everyone in diagnostics, looks like a picture of mental health.
however, if you are as insane as I am about kutner, you could notice that kutner was obviously not as alright as he claimed he was, as alright as all the characters thought he was. and as for my personal opinion, I do think you could make the argument that kutner was kind of passively suicidal from the moment we meet him, this being one of the reasons behind his reckless and borderline self-sacrificing behaviour (example: the charged paddles incident in mirror mirror), but, again: kutner's suicide was not planned, or meant to be foreshadowed. another opinion I hold is the distinction that kutner was meant to be secretly depressed/mentally ill, but not suicidally depressed, and if kutner hadn't needed to be written off we would have eventually gotten some kind of storyline showing this.
what I'm trying to get at is that kutner's death is meant to drive the story forward, it's meant for (mostly) house and (somewhat) the other characters to react to, it's meant to be the final nail in house's grief-shaped coffin that worsens his addiction and thus, makes him start hallucinating. hell, we even meet his adoptive parents in simple explanation. kutner's suicide, to put it simply, is about everyone but himself. kutner's death isn't about him.
