Perhaps it is the many brutal wars in which Senwosret (aka Senusret) fought that gave his face a somber appearance.
Senwosret III's statue at the Brooklyn Museum has a less damaged nose:
Sphinx of King Senwosret III
Gneiss
Dynasty 12, reign of Senwosret III (ca. 1878-1840 BCE)
Egypt, From Thebes, Karnak temple
Gift of Edward S. Harkness, 1917 (17.9.2)
Sadly, his nose has been damaged, and it looks deliberately gouged.
From info card:
"The crouching feline body of this sphinx, with its royal bull's tail, is remarkable for the way in which the veins of the stone underline the body forms, as if the sculptor wished to negate the boundaries between inert nature and the animal world. The brooding face, on the other hand, is not without allusions to the bestial. the great historian Cyril Aldred has called it the 'haunting portrait of an autocrat.' During the reign of Senwosret III, sculptors freed themselves from the idealism that previously dominated most representations of the pharoah and started to explore without reservations the
physical appearance of flesh and bone in a ruler's face."
There's a sense of his seriousness and strength in this able portrait of him as an older ruler.