I think it's a natural thing, after all HN is also the 'breedingground' for a lot of YC startups, and some of the emotions around applying for, acceptance and rejection are expected to spill over into the forum.

The tips by those that 'got in' serve mainly to reassure those that are about to make their pitch, which, if you've never done it before can be absolutely terrifying.

Whether or not you get in, you always learn something.

I think because the barrier to entry is so low at YC that you'll see a lot more of this emotion than you'd expect in a more seasoned environment, for many people this is the first time ever they apply anywhere, and given the number of applications vs the number of available slots it's logical that a bunch of people with an excellent plan and team are not going to be funded.

That can be quite frustrating. Give it a few days everybody will have picked up the pieces and moved on, and they'll start realizing (if they haven't already) that not being accepted by YC is not the end of the world.