The inability to be completely and totally silly or to respond sympathetically to such reveals a dead or dying spirit. Of all the pretenses available to most, to be utterly silly might also be among the hardest responses to fake.
The imagination is not about picturing things. Even if it is, this is a small, unimportant part of it. Imagination is more about believing in possibilities where none had existed before. But accepting limitations might assist in this. Knocking one's head against the wall is bad for the mind so it is equally bad for the imagination.