It is a mistake to take tranquility to mean perpetual state of bliss. Since being Muslims does not make us immune to tragedies and catastrophes. What a state of tranquility does is to empower us to handle life's difficult moments with our spouses as obedient servants of Allah. Allah in His infinite Mercy also provides us with the tools by which we can achieve this state of peace and tranquility. We are therefore inclined by our very nature to have mercy for our spouses.
Mercy is manifested through compassion, forgiveness, caring and humility.
It is obvious that these are all ingredients that make for a successful partnership. Marriage in Islam is above all a partnership based on equality of partners and specification of roles. Lack of mercy in a marriage or a family renders it in Islamic terms dysfunctional. Allah further states that He has also placed in addition to mercy, love between spouses. It should however be noted that Islamic concept of love is different from the more commonly understood romantic love so valued in the Western cultures.