I love this line. But I just want to express that on the other hand everything in the universe needs and seeks proper balance.

We have come out of balance. We became too serious where we needn’t, and too careless where we should be more attentive.

But I also beleve we are exactly where we need to be.
To start seeking balance we first have to experience (enough) imbalance.

So, ‘don’t be too serious’ is as relative statement as we need it. The same as ‘don’t be too careless’.