What can make couples loose interest in each other?

Simple, both were never fully-invested from the beginning and only thought of their own self.

Poor communication makes partners drift apart.

Feeling unappreciated kills interest.

Lack of affection or intimacy creates distance.

Routine and boredom reduce excitement.

Unresolved fights cause resentment.

One person giving more than the other causes imbalance.

Trust issues damage connection.

Growing in different directions pulls couples apart.

Stress and personal problems affect relationships.

Taking each other for granted weakens love.

I think lack of respect for yourself and the other person as well as lack of communication.