Most of the time, people will straight up tell you what they are seeking and what the purpose of their OP is. I see a lot of topics where the OP says something like "i made this mistake /did something wrong; now what should i do?" and then a pile-on of people berating the OP for what they already did, lecturing them on how stupid they were, etc. How is that helpful? It's not. Then when the OP says it's not helpful and not what they asked for, everyone gets all "but you asked for advice! this is the internet, i can say anything i want; don't say anything if you don't want my unhelpful word vomit in reply!"
If the OP says they're already beating themselves up and have regrets and they are looking for encouragement and ideas on how to move forward rather than another useless lecture, and then you give them a useless lecture, come on! Have some self control and if you don't have anything actually helpful to say, don't bother.
Another frequent one is an OP asking for advice on how to do something (transition into a diffcult-to-enter specialty area without a lot of experience, get a hospital job as a new grad, get a new job with a less-than-stellar employment history, etc.) and getting replies of "you can't". Fail! Not helpful. No wonder people started putting "no negative replies please" in their topics. The OP didn't ask for a chorus of negativity; they want to know if anyone has any creative, out-of-the-box ideas for accomplishing what they want which they may not have considered or may not know about. If you can't give someone what they're asking for, then you don't have anything of quality to contribute and you should click out of the topic without saying anything.
Same with topics about conflicts with a manager, co-workers, and/or patients. Assuming the problem is really the OP and replying as such is useless. If you can't put yourself in the OP's shoes, or you feel too close to the issue (i.e. if you're sensitive about negative attitudes toward older nurses and someone complains about a co-worker who is older), check your biases before you reactively accuse the OP of being the problem.