Mutual Love
Mutual understanding
Mutual Respect
Mutual trust
Mutual care
ect .. .
These are first essentials in my opinion. As stands & foundation of a relationship. Without them the relation can be exposed easily & would be vulnerable. With them you can start a calm one which only it's a start, in order to continue there gonna be other factors to keep the relation going. (some factors are general, some factors are the one you need to discover individually about the person you're with)
However, so many starts a relation without having all of the values above! But yet you can see they could finally work it out! That happens when a person share their lives with someone else while they struggling with their own lives yet! & haven't found themselves in life. So when they get to be together. They struggle until they develop their lives & find their zone. But in these circumstances, the risk of breaking up is high.