It doesn’t take too much imagination to extend his analogy to human beings. In fact, the worst horrors that humans inflict on other humans make being eaten by a lion seem tame in comparison.
Imagine something you really like. Be it gourmet food, music, sex, anything. You get a 25 year supply of it, but you have to agree to an hour of waterboarding. I bet you wouldn’t make it 5 minutes before you backed out of the agreement. This is what Schopenhauer is talking about: the asymmetry of pleasure and pain. However good the best pleasures are, the worst pains are so much more horrific.
I’m don’t agree with that synopsis. Sure, horrible horrors exist. But wonderful pleasures exist as well. I’d guess that people who do pessimism for a career haven’t experienced the best pleasures that life has to offer, which is a pity. The pleasures still exist, though.
The broader point does not rest on comparing the worst horror to the best pleasure, however. It rests on observing that for most people, most of the time, life is pretty good. For me, today, I’ve not suffered any noticable pain and I don’t expect I will. I’ve had a number of pleasures: eating, drinking, driving, working, and so forth. So on average, pleasure is beating pain.
Your “obvious” analogy is nowhere near as obvious as you think. Lots of people willingly endure temporary suffering in order to obtain something that will make them happy. People are willing to endure pain for things as trivial as a ear piercing to as important as childbirth.