Allah commands His servants to be dutiful to parents, after urging them to adhere to belief in His Tawhid, because a person's parents are the cause of his existence. So he must treat them with the utmost kindness and respect, his father for spending on him and his mother because of her compassion for him. Allah says:

وَقَضَى رَبُّكَ أَلاَّ تَعْبُدُواْ إِلاَّ إِيَّـهُ وَبِالْوَلِدَيْنِ إِحْسَـناً إِمَّا يَبْلُغَنَّ عِندَكَ الْكِبَرَ أَحَدُهُمَا أَوْ كِلاَهُمَا فَلاَ تَقُل لَّهُمَآ أُفٍّ وَلاَ تَنْهَرْهُمَا وَقُل لَّهُمَا قَوْلاً كَرِيمًا - وَاخْفِضْ لَهُمَا جَنَاحَ الذُّلِّ مِنَ الرَّحْمَةِ وَقُل رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِى صَغِيرًا

(And your Lord has decreed that you worship none but Him. And that you be dutiful to your parents. If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor shout at them, but address them in terms of honor. And lower unto them the wing of submission and humility through mercy, and say: "My Lord! Bestow on them Your mercy as they did bring me up when I was young.") (17:23-24) Although Allah orders us to show kindness, mercy and respect towards them in return for their previous kindness, He says:

Commentary

وَوَصَّيْنَا الْإِنسَانَ (And We have instructed man - 29:8). Wasiyyah (وَصِیَّۃ) means to call someone for some action, when the call is based on advice and well wishing. (Mazhari)