really ill...?

when people are diagnosed with certain 'mental' illnesses, regardless of the 'degree', what are the decisions based on?
who decides what is illness and what's not?
in these people, doctors may find patterns in the individual and maybe across a group of individual and long behold a new term is added to the world of diseases/illnesses.

but... couldn't it be, sometimes, just a lack of islam?

Surat Taha: قَالَ اهْبِطَا مِنْهَا جَمِيعًا بَعْضُكُمْ لِبَعْضٍ عَدُوٌّ فَإِمَّا يَأْتِيَنَّكُم مِّنِّي هُدًى فَمَنِ اتَّبَعَ هُدَايَ فَلا يَضِلُّ وَلا يَشْقَى {123} وَمَنْ أَعْرَضَ عَن ذِكْرِي فَإِنَّ لَهُ مَعِيشَةً ضَنكًا وَنَحْشُرُهُ يَوْمَ الْقِيَامَةِ أَعْمَى {124}

couldn't islam help with the most part, by providing people this great sense of understanding of life, and its goals?? wouldn't that instill some positivity to people diagnosed with for example, depression and other illnesses?
wouldn't that at least add some meaning to life, or even prevent it in the first place... maybe reduce the severity? i have reason to believe so.

Allah created us for a reason, and it seems like some people either want to escape the responsibilities in life, or deem almost half our population of illnesses that will, 'legally' opt them out of such responsibilites

فَمَن يُرِدِ اللهُ أَن يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلإِسْلاَمِ وَمَن يُرِدْ أَن يُضِلَّهُ يَجْعَلْ صَدْرَهُ ضَيِّقًا حَرَجًا كَأَنَّمَا يَصَّعَّدُ فِي السَّمَاء كَذَلِكَ يَجْعَلُ اللهُ الرِّجْسَ عَلَى الَّذِينَ لاَ يُؤْمِنُونَ {125} Surat alAn'aam.

to nonmuslims doctors, think of this ayah in surat ali Imraan
{إِنَّ الدِّينَ عِندَ اللهِ الإِسْلاَمُ وَمَا اخْتَلَفَ الَّذِينَ أُوْتُواْ الْكِتَابَ إِلاَّ مِن بَعْدِ مَا جَاءهُمُ الْعِلْمُ بَغْيًا بَيْنَهُمْ وَمَن يَكْفُرْ بِآيَاتِ اللهِ فَإِنَّ اللهَ سَرِيعُ الْحِسَابِ {19}
This knowledge of the dunya that you possess, of the human body, and all the conflicts across medicinal aspects... why does it exist!?

surat alMaa'idah وَرَضِيتُ لَكُمُ الإِسْلاَمَ دِينًا فَمَنِ اضْطُرَّ فِي مَخْمَصَةٍ غَيْرَ مُتَجَانِفٍ لإِثْمٍ فَإِنَّ اللهَ غَفُورٌ رَّحِيمٌ {3}
So Allah has outlined for us even what will cause us physical damage, and people still don't accept this truth... and still fall ill because of going against the 'solutions' in this deen!

surat azZumar أَفَمَن شَرَحَ اللهُ صَدْرَهُ لِلإِسْلامِ فَهُوَ عَلَى نُورٍ مِّن رَّبِّهِ فَوَيْلٌ لِّلْقَاسِيَةِ قُلُوبُهُم مِّن ذِكْرِ اللهِ أُوْلَئِكَ فِي ضَلالٍ مُبِينٍ {22}
...and we wonder why there are so many heart diseases..!