It is common knowledge that the top of your bed is the only safe haven from the infamous monsters that live beneath it. A bed is a relatively small space, so those monsters can keep track of your every move. That's bad enough. But monsters are second worst. What is first? Spiders. Spiders are the worst scary things, simply because there is nowhere you can go where they cannot get you.
Say you are in bed, under siege from the sub-mattress ghouls as usual, but still, you're feeling pretty safe. Then BAM! Spider drops on your face from the ceiling! In your panic you claw at your face, and soon the arachnid releases its grip on your skin. By now you've recovered enough to acknowledge that this issue must be dealt with. But when you look for the spider again, of course it has vanished.
Or possibly you are feeling brave enough to venture into Snatching Area (also known as the floor of any room containing a bed). Ogres & Co. are leaving you alone (probably they already caught your dog or something and are busy eating it) and you are feeling pretty confident. Maybe you just brushed your hair, so when you feel that tickling sensation on your leg, you assume it is just a stray lock. But when you brush it off, your fingers come in contact with something significantly larger than expected. As you may have guessed, it was a spider. And this ends the same way as Scenario #1.
Personally I find that spiders pose a bigger threat to my mental health than my physical well-being. I get so paranoid that I can rarely sleep after an attack, and I am left telling the tale of my misfortune to the inhabitants of the Under Bed, who are very understanding and offer both condolences and hugs (of course I have never accepted the latter, otherwise I wouldn't be writing this right now. I would be dead.)