This comment is obviously true but uninteresting if you don't elaborate further on those causes

What I mean is, the comment you replied to isolated a specific cause and sparked a discussion; your comment, if taken at face value, is thought-terminating. How can we possibly comprehend all causes of complex phenomena before we are allowed to discuss them?

About the universally true thing, I understood it as whether people that's unhappy with life generally have trouble sleeping, not whether everyone that have trouble sleeping is unhappy with life. Still probably not an universal but is more reasonable sounding