Routine in your relationship can be a problem, but it does not have to be. For some people it makes the relationship easier and more stable.
Mental or physical difficulties or a disease can lead to relationship problems.
Relationship problems often lead to sexual problems, or the other way around.
If you have relationship problems, you can try to talk with your partner or someone you can trust. If necessary, you can also look for professional help. It takes time to solve a relationship problem. If you cannot overcome the problems after working on it for a long time, you can consider splitting up, or getting a divorce if you are married.