The problems without an obvious solution, or the problems where you and your partner disagree in terms of the solution or the existence of the issue.
Those are the problems that tend to destroy a relationship. Every relationship runs into obstacles… but if you can work together and be honest and equitable… most obstacles can be overcome through work and compromise. But only in that way. If you can’t agree, can’t find compromise, can’t be honest and equitable in effort… the obstacle ruins the relationship.
External factors like finances. Financial strains can destroy relationships because obviously you need to survive and want to thrive. Also, internal factors like how the two of you as separate beings are on different paths in life, or someone is straying from the relationship (cheating or flirting with that temptation, or just withdrawing from the emotional relationship a bit even…)