Sometimes because they get defensive in the face of an unvarnished, less soft focus view of the issue they are seeking input on. Change is hard. And can be scary. Smearing vaseline on the lens and getting shirty with the people wiping it off is a classic mode of avoiding change. They wanted a vent, and got a sharp forcus image of their life instead. Which makes them backtrack at warp speed nine.
Sometimes because they wanted a vent about a pretty normal life hiccup and some posters are determined to reframe said life hiccup as a life Earthquake + Tsunami. Then run away with a few posted details to create their own imaginatively embellished context to place them in, all while insisting they know the OP (Raging Disaster !) of a life better than the OP does. OP gets v. frustrated and attempts to bring the details back into a rather more "connected to my actual reality" context than the one the doomsdayers are promoting.
And some other scenarios that fall between the two probably.