I think on the topic of shipping, it simply boils down to letting Jeph be the storyteller. If two characters are canonically linked romantically, then obviously discussion and speculation is okay. And, if you can make a decent argument that the plot is hinting at two characters hooking up, that's suitable speculation, as long as you don't grab the idiot ball and run screaming into R-Rated territory with it. This is, after all, a subforum about discussing QC and its characters, and if the plot is actually headed in a certain direction, there is absolutely no harm in talking about it.
But there is an extremely clear difference between this, and people who say that "A and B should hook up because they would be sooooo cute together!" or "A needs to get laid to overcome their problems, and B shoud be the one to do it!". Let alone the extreme examples of "Hurrr durrr A and B need to have sweaty sex because boobies durrr." And we all know perfectly well that examples of these have popped up many, many times before. There's no plot-related reason for it to happen, nothing in the comic that suggests it ought to happen, no reason to think it's likely to happen. It is, as pwhodges said, shipping merely for the sake of shipping.
In the Marten-Hannelore example? They're close, they spend a lot of time together, they talk about personal topics - but these are all things that very close friends would do anyway. Hannelore, however, has not actually shown anything resembling romantic interest in Marten unless her initial "stalker" phase could be considered that. And Marten became openly disgusted at the suggestion he views Hannelore in a sexual light when Dora and Faye asked him which of his female friends he fantasizes about.
There is less canonical evidence to suggest a romantic link between Hannelore and Marten than there is between Marten and Steve - the two guys share a relationship just as close as Marten and Hannelore do, but they have also been mistaken for a couple, and have canonically had drunken makeouts. But I don't think anyone here is insane enough to suggest that Marten and Steve are likely to hook up soon, are they? Even without Jeph putting the Kai-bosh on speculation of Hannelore's love life, the pairing makes exactly zero sense anyway in the context of the story.
Shipping for the sake of shipping is non-canonical discussion, which is off-topic on the WCT anyway. It's also trying to control Jeph's story, which is pretty rude and understandably irritating for the guy. But most importantly, since he's very specifically said he doesn't want it here... asking why is a completely irrelevant question. Maybe he has a good excuse. Maybe he has a bad one. Maybe it's an insane reason that involves turnips, squirrels, and a beret. The reason does not change the fact that we've been told not to do it, and around here, he has the authority to make that rule. So, if you actually respect the guy as a writer, that's what you do.