Seriousness in play is a statement that is at the least seemingly profound due to its paradoxical nature. Like all paradoxes, we tend to give them our attention even to an absurdly desperate point. Whether this is in an attempt to understand them fully, seek out different variations of perception perhaps, or even hammer it down so it becomes a mantra.
The best way to understand seriousness in play that I have found is via the pairing and contrasting of the terms complex and simple. Pair complex with seriousness, while simple is with play. This implies that all things serious are complex, whereas all things of play are simple. We can even observe this in regard to two different expressions in the current moment. When politicians debate and argue over things that may be nothing but nonsense, there is almost always an induced tone of seriousness that urges the seeming complexity of the matter at hand. On the flip side, we see with play that play itself is simple, and when it gets too complex, the essence and nature of play can become fragile and broken down. Perhaps best understood via two children playing a game of house or even tag. House has more rules than tag, yet, the complexity of the game, the simplicity itself is determined via the rules put forth. If one is to play a lax game of house, then it is simple. Going further, if one is to play a true game of tag, it is even simpler.
Thus, we can best understand seriousness in play via applications such as virtue, if we take things like humor and even wittiness as true virtues. If one is serious about being virtuous, they can ensure that the proper virtues are retained so that play is witheld and put forth. If one is not serious about virtue, so long as we still accept that humor and wittiness are virtues themselves, it would be true that the rejection of the aforementioned two would provide a prioritization of seriousness over play. In this variation, we could argue that one does not understand how to apply the "golden nugget" that is seriousness in play.
Regardless, it seems that to avoid a paradox, we must be willing to understand seriousness in play like a folder. The folder may be labeled "all that is serious", yet, within contain notes on how to be full of reverence towards those that deserve it, along with a few jokes and clever punchlines that allow for play itself to exist within the folder of "all that is serious", even if solely for the attempt at creating a smile or engaging in friendliness.