Why is the poet unhappy with God in "On His Blindness"?

The speaker is initially unhappy with God for making him blind; he thinks this means he'll no longer be able to serve him. He's spent nearly half his life serving the Almighty through his God-given gift of writing, yet that gift has now been cruelly snatched away from him due to the onset of blindness, and the speaker can't understand why. Why should someone who's been such a loyal servant of God be treated like this?
But patience is a virtue, and the personified figure of Patience comes to Milton in his hour of darkness (both literal and metaphorical). Patience tells Milton that God doesn't need any special works from man, such as his writings. He will always love best those who "bear his mild yoke," i.e. those patiently accept that God is in control of their fate. Even those, then, like the blind Milton, who are unable to rush "o'er land and ocean," can also serve God just as well though their patient bearing of life's misfortunes.
https://www.poetryfoundation.org/poems/44750/sonnet-19-when-i-consider-how-my-light-is-spent