When two people are young and dating each other they might not be ready to settle down, and their agreed upon goal might be to just date casually. The relationship could become so comfortable that neither party sees how fast the years are passing, but the timing could eventually become an issue between them. Many people are not ready to settle down and build a family or future when they are young, but looking at it from the standpoint of age comes at different times for each person. The one who eventually wants a commitment might leave when they do not get it, and their former partner will spend a lifetime talking about the one who got away.
It might be that two people do not want a commitment when they meet, but dating a person for several years could change the mind of at least one of them. People who suddenly realize they have found the person they want to build a future with are often willing to broach the subject, but their partner might tell them they are not ready. The unwilling partner might ask them to wait another year, or they could tell them there will never be a future for them as a couple.
For the one who is not willing to consider a change in their relationship, selfishness and insecurity are often the roots of their stubbornness. They see their life as something they do not want to share with someone else, and they are often afraid that making a commitment would make them vulnerable. Giving their partner an ultimatum to maintain the status quo or leave is often their only solution, and they are the ones who will be left behind while their partner moves forward in life.
It can be difficult to change the status of a relationship that has weathered years as a casual dating scenario, but it can be done if both people are willing to make the change. Those who would rather brave out their current status often find their partner was serious, and they would rather leave a working relationship than forego a real commitment for the future.