There's a cyclical nature to relationships. You have good times and bad times, good days and bad days. That's normal. It's only a problem when that wheel stops turning and the relationship stagnates. When things no longer change in any way, when there is no growth and no motivation to move forward, that's the time to get off the bus and start walking away.
We tend to look at other people's relationships and see all the happiness or traits that our relationships lack. We tend to forget that people very seldom show up in their messiness especially when they're representing their relationships. That's why break ups are so often a huge shock to friends and family.
To continue with that wheel analogy, a relationship is very much like driving a car. Sure, you need to keep an eye on other drivers to make sure they don't damage your car. It's far more important though to pay attention to the road and your own driving. If your exhaust falls off, fix it. Wear a safety belt to prevent serious injury and make sure your passengers are secure and safe too.
Relationships are about journeying together and if you want that journey to be long and pleasant then it's always a good idea to allow your focus to fall not just on yourself but on the people travelling with you.
And like any good car, service that damn relationship, keep it clean, take care of it and trust that it's going to keep you safe and secure in return. It's about give and take and it's about choices. It's about recognising the value and treating it with the respect and care it deserves.