Wonder if this true?

I tried briefly to look this up on line. Do not know that it is true, but gave me pause for thought. I don't usually look to James Patterson and Maxine Paetro for my life philosophy, but they wrote in The Sixth Target, quoting a physician:

"Claire once told me this thing about brain chemistry, the nub of it being that when you're feeling good, you can' t ever imagine feeling bad again. And when you're feeling bad, it's impossible to imagine a time when you won't be circling the drain."

I find parts of that true. When I have been through a bad spell, I swear I will be eternally grateful for the simple things I used to do with no thought of what it took to perform those things. Then when my strength returns I eventually give no thought to 'taking things for granted' including the things I could not do earlier. Do I fail to give thanks more often because I cannot imagine losing strength and "mundane abilities?"

I need to keep all that in balance.