The Messenger grieving by the Statements of the Polytheists, and His Gratification

This statement of Allah, the Exalted, to His Messenger comforted the worries that the polytheists were causing him due to their statements directed towards him. This is just as Allah says about them,

وَقَالُواْ مَا لِهَـذَا الرَّسُولِ يَأْكُلُ الطَّعَامَ وَيَمْشِى فِى الاٌّسْوَاقِ لَوْلا أُنزِلَ إِلَيْهِ مَلَكٌ فَيَكُونَ مَعَهُ نَذِيراً - أَوْ يُلْقَى إِلَيْهِ كَنْزٌ أَوْ تَكُونُ لَهُ جَنَّةٌ يَأْكُلُ مِنْهَا وَقَالَ الظَّـلِمُونَ إِن تَتَّبِعُونَ إِلاَّ رَجُلاً مَّسْحُوراً

In the cited verses, the Mushriks had demanded the kind of miracles they chose. Verses that follow tell them that they already have a miracle, the miracle of the Qur'an shown at the hands of the Holy Prophet ﷺ before them, a miracle even they could not deny. Now if they were demanding these miracles honestly, only to determine the veracity of the messenger of Allah, then, their demand stands accepted (and the miracle is on). And if this demand is simply to gratify their hostility, the showing of miracles demanded by them would remain redundant, for who could expect from such hostile people that they would embrace Islam, even after having seen those miracles. So, the essential point is that the Holy Qur'an is a clear miracle that cannot be denied.

A refutation of the false doubts engineered by Mushriks and Kafirs in it has been made in the next two verses (13, 14): These people say that the Prophet of Islam has himself made up the Qur'an - it is no book of Allah.