The reason for that request was that when Moses, peace be upon him, listened to the words of Allah the Almighty and was delighted, he wanted to have such a feeling by seeing the Face of Allah the Most Generous. Allah, Exalted and Sublime be He, told him that due to the limited nature of the worldly life, no one can see Him. If Allah were to manifest His Divine Character to man, the latter would perish like the mountain.
(When Moses came at the appointed time and his Lord spoke to him, he asked, “My Lord! Reveal Yourself to me so I may see You.” Allah answered, “You cannot see Me! But look at the mountain. If it remains firm in its place, only then will you see Me.” When his Lord appeared to the mountain, He levelled it to dust and Moses collapsed unconscious. When he recovered, he cried, “Glory be to You! I turn to You in repentance and I am the first of the believers.”) (Al- A’raf 7:143)
سبب الطلب أن موسى لما سمع كلام الله و تلذذ بخطابه أراد أن يتلذذ برؤية وجه الله الكريم فأخبره الله عز و جل أن حكمه في الدنيا أنه لا يرى و لو تجلى الله للإنسان حتى يراه لهلك الإنسان كما تحطم الجبل.
جَاءَ مُوسَىٰ لِمِيقَاتِنَا وَكَلَّمَهُ رَبُّهُ قَالَ رَبِّ أَرِنِي أَنظُرْ إِلَيْكَ ۚ قَالَ لَن تَرَانِي وَلَـٰكِنِ انظُرْ إِلَى الْجَبَلِ فَإِنِ اسْتَقَرَّ مَكَانَهُ فَسَوْفَ تَرَانِي ۚ فَلَمَّا تَجَلَّىٰ رَبُّهُ لِلْجَبَلِ جَعَلَهُ دَكًّا وَخَرَّ مُوسَىٰ صَعِقًا ۚ فَلَمَّا أَفَاقَ قَالَ سُبْحَانَكَ تُبْتُ إِلَيْكَ وَأَنَا أَوَّلُ الْمُؤْمِنِينَ
الأعراف 7: 143