Why we should follow quran and sunnah ?

Good that you get the point, let me now explain it to you using another perspective.

I don't know about India but in the West for some exams we have negative marking and here is a difference between marking:

1. Positive Marking: If you make a mistake you don't get anything deducted
2. Negative Marking: If you make a mistake then marks get deducted

You are absolutely right that 5 times daily Salah is what is required from you and Sunnah/Naf'l are optional and I agree with you. Problem is that you are REQUIRED to make PERFECT 5 times daily Salah, can you do that? OR at least PASS!

If you can't do that then marks from Sunnah/Naf'l prayers are added to make up for your deficiences in the ORIGINAL FAR'DH.

Insha'Allah we will not be negatively marked but it will be VERY DIFFICULT for us to become perfect in our daily Fardh Salah so reading Sunnah/Naf'l does two things:

1. IF our Fardh paryers are perfect they get us bonus marks
2. IF our Fardh paryers are NOT perfect they get us marks which can be made up towards the deficient prayers

And my brothers, I am the most weak, feeble and deficient in my prayers so I need my bonus!

central-mosque.com/index.php/Acts-of-Worship/sunnah-moaki...

1. DON'T EAT and also believe that you will be fed without you moving
2. DON'T WEAR WARM CLOTHES and also believe that you will be kept warm
3. DON'T SEE A DOCTOR and also believe that you will be cured

But if you don't do 1,2 & 3 then it is illogical for you to apply this sort of thinking to your prayers and good deeds only.

The same Allah (SWT) who says that I give you Tawfeeq for good deeds also says that I feed you!

Because Nabi (Sallallaho Alaihe Wassallam) acted in this manner:

1. Nabi (Sallallaho Alaihe Wassallam) always prayed his Fardh Salah
2. Nabi (Sallallaho Alaihe Wassallam) always prayed some of the Sunnah Salah
3. Nabi (Sallallaho Alaihe Wassallam) sometimes prayed some (optional) Salah and sometmee he missed it
4. Nabi (Sallallaho Alaihe Wassallam) only prayed Eclipse prayer during Eclipse
5. Sahabah (RA) only prayed rain prayer during drought

How can 1,2,3,4 & 5 be in the same category? Unless and until a person is well versed in  Qur'aan and Sunnah he has no way of distinguishing and differenetiating 1,2,3,4 & 5.

The Fuqaha have done it to make it easy for people.

• The crime for RAPE and LITTERING in India are different so one may ask both are crimes why are they punished differently?
• OR the reward of dying while fighting for India and volunteering to help the poor or needy are different so one may ask both are crimes why are they rewarded differently?

Our  whole life is based on categorisation, during Exams we don't play as much and devote more times to study and revision while during Holidays we don't study as much (or don't study at all) so we all prioritise!