Having a relationship with someone is different to having a marriage with someone. Some people seem to have a lovely caring relationship when they are dating , even if it is long term dating, but as soon as they get married it becomes a different dynamic.
Mostly we get to find out these things only in the negative situations.
Frustrations or arguements seem uncontrollable, finances get affected and overall mindset is disturbed. But this was never the case when with the same person before marriage.
This is because the actual act of marriage activates the Navamsa chart and indeed the 2nd, 5th, 7th, 8th and 12th houses.
2nd- bank balance and extended family
8th- happiness of marriage
12th- physical happiness/comfort
If there are malefic planets in the 7th especially then can result in disturbance to marriage. However 5th house malefic planets also causes a distance between the spouse and the natural affinity towards them. So it stops person making an effort to maintain relationship. The situation becomes such that two people are living together but have no connection.
Malefic influences of certain planets on the 7th and 5th houses can invoke the following situations
SUN – ego clashes
MOON – manipulation
Mars – domination/ abuse
Mercury – false promises / cheating/ split personality
Jupiter – financial losses, arguing over money, different perspectives on living or differences on following family traditions
Venus – Cheating, ego clashes, temper
Saturn – lack of feelings/ attraction, distance, stress not making efforts , cheating
Rahu – volatile arguements, issues with trust, posessiveness, abuse, controlling behaviour
Ketu – hating each other due to being unsatisfied or unfulfilled expectations, differences with family, not caring, feeling of instability
It is not as simple as just looking at what planet is where. There is more required in terms of aspects of other planets, strengths positions, and glances at divisional charts too.
But in general, a relationship changes after marriage and it is because of the activation of other locked areas. If these areas have good influences then relationship flourishes, else it changes to need more efforts from both people.