Marriage is a covenant of love which unites husband and wife to form a family unit. This covenant is sealed by God. Therefore, no one can break this covenant. Furthermore, they are no longer two, but one flesh. (Matt. 19:6). The relationship between husband and wife therefore is stronger than that between a father and his son or between a mother and her daughter. Therefore, even if one divorces one’s spouse, he/she is still remains his/her spouse until death. Furthermore, it does not matter how bad one's spouse is, he/she is still that person's spouse; just as irrespective of how bad one's son is he is still that person's son. Therefore, we have to love our spouse just as Christ loves His Church (Ephesians 5:25) and forgive one another without any limit (Matthew 18:21-22); just as God forgives us. Because of our sinfulness there are always going to be stressful situations in a marriage. However, Jesus who came to heal us gives us strength and graces to live a Christian marriage. Therefore, when we follow Jesus by denying our very self and taking up our cross (Matt 19:11) we will find hope where there is hopeless, love where there is hate, peace where there is discord, and see the face of God in the pain and suffering of our spouse.
The Catechism explains it very well (this is only a small taste):
**1660 **The marriage covenant, by which a man and a woman form with each other an intimate communion of life and love, has been founded and endowed with its own special laws by the Creator. By its very nature it is ordered to the good of the couple, as well as to the generation and education of children. Christ the Lord raised marriage between the baptized to the dignity of a sacrament (cf. CIC, can. 1055 § 1; cf. GS 48 § 1).