Marriages are made in heaven.. and not every relationship is built to last forever. Some of course are built for eternity I say. While some break with time.
Not every relationship is perfect! Even a good and stable one can end
There are several ways for a relationship to breakdown like
Communication in a relationship should always be clear, bold and interactive. And the couple should make it a point it should be moderate. If it's over the top and extreme, where you expect your partner to over analyse things like you do, that can pose as a threatening situation. The lesser and clear the interaction the happier and healthy would be the relationship.
Lack of trust: where there is no trust a relationship can never exist between two people. The trust between two people can diminish due to many reasons it could be that the have lied or cheated at some point of time or due to their past relationships. This is the major trait of any healthy relationship.
Once this collapses there exists nothing in a relationship. First, to make the relationship better they need to work on the building the trust between both of them.
Abusive nature: abuse in particular need not be physical it can be mental emotional abuse too which equally toxic as physical abuse. Which will make the relationship fall apart. The abuse have to end at some point or else the relationship will take apart.
Jealous nature: this trait is interlinked to insecurity in life which will certainly lead to failed romance . You could be jealous of the company your partner keeps to even them more successful than you. If you see your partner is having a problem if you talk to a friend. Then it's absolutely splitsville from there on