Well, worrying and regretting comes from knowledge. Ignorance could be argued to be bliss in some ways as for example let's say you receive a grade B in a test which you are happy with. You demand to see the marks of your test and which question you got wrong, turns out if you'd have known one more point you'd have gotten an A grade. But if you hadn't demanded to know the results, you could have been happy with your B, but now you're going to regret not knowing that one piece of knowledge because you decided to know your marks, and if you don't get in to university you're probably going to still regret it, etc etc. On the other hand, sometimes all you need is some knowledge and your problem is solved, so I guess that would be why ignorance is not bliss.
It's just another way of saying "What you don't know can't hurt you."
That's probably a bad example and a poorly worded argument, sorry. But take what you can from it. I'm not the best with English or debates, but I need to brush up my skills for my philosophy course.