Whether, and how much, you trust your own judgment over that of an expert should depend at least in part on the degree to which you think your situation is unusual.
The expert also is better equipped to discern whether a situation is unusual, because the expert has seen more.
To the non-expert, something really mysterious and weird must be going on to explain these puzzling symptoms. Computer A can ping computer B, but B can't ping A? That's so strange! After all, ping is supposed to test whether two computers can talk to each other on the network, right? How could it possibly work one way but not the other? Is something wrong with the switch? Is one of the network cards broken? Is it a virus?!
To the expert, that's not unusual at all. One computer has the wrong subnet mask set. Almost every time. Like, that's 20 to 100 times more likely than a hardware problem or something broken in the network infrastructure, and it can be checked in seconds. And while the machine may have a virus too, that's not what causes these symptoms.
The expert also is better equipped to discern whether a situation is unusual, because the expert has seen more.
Very true as well, though I will add the counter-caveat that the expert is usually biased toward concluding that your situation is not unusual. This is why many "tech support horror stories" have a bit where the narrator goes "... and then, when they finally got it through their heads that yes, I had tried restarting it five times, and no, I didn't have the wrong settings ..."
Another month has passed and here is a new rationality quotes thread. The usual rules are: