وَمِنْ آيَاتِهِ أَنْ خَلَقَ لَكُم مِّنْ أَنفُسِكُمْ أَزْوَاجًا لِّتَسْكُنُوا إِلَيْهَا وَجَعَلَ بَيْنَكُم مَّوَدَّةً وَرَحْمَةً ۚ إِنَّ فِي ذَٰلِكَ لَآيَاتٍ لِّقَوْمٍ يَتَفَكَّرُونَ
And among His Signs is this, that He created for you wives from among yourselves, that you may find repose in them, and He has put between you affection and mercy. Verily, in that are indeed signs for a people who reflect.
[Quran ar-Rum 30:21]
"that you may find repose [taskoonoo] in them".
The word in arabic for Repose is; Sakoon.
Sakoon = Tranquility/calmness. Anyone who has been in love has felt this strongly when they are with their lover. This is one of the strongest feelings that person will feel when they are with their lover.
This feeling of joy, contenment and tranquility makes the person have extreme love for that person. So Allah after explaining the first attribute of Tranquility says describes the next stage;
"..and he has put between you affection (Mawaddah).."
Mawaddah is love with a passion, an extreme liking to something.
So the lover;
1) Feels at ease [feels calmness] with the person.
2) Has an extreme passionate liking to them.
This Mawaddah stage usually lasts for around 2 years.
As the couple continue living together, the passion gradually lessens. Both partners mature abit more and the fantasies die out abit more.
So many obstacles come within their way because they are living with each other. Before they were both blinded by passion, but now they see each others mistakes and errors.
So Allah tells us;
"and He has put between you affection and MERCY (rahmah).."
So now that the passions have slightly died out, and the calmness isn't always there in the relationship (arguments do happen in marriage) - Allah tells us that He has placed between the lovers Mercy.
So even though they might argue with each other, they have mercy on each other and forgive. This mercy keeps your relationship going, because deep down inside you still care that your partner doesn't get hurt.
"..Verily, in that are signs for people who reflect."