All relationships start with growth no matter how long or brief they are. You are attracted to someone simply because of the opportunity for growth that they represent to you.
How long the relationship lasts depends only on how much growth opportunity is there. If you are not growing you are dying. Therefore, fear of death is the end of all relationships.
Once you've stopped growing with a person, you've begun dying with them. In order to avoid death, you end the relationship. Fear of death is the root cause of all negative emotions and thoughts.
So how do we overcome our fear of death?
You feel excited by the new attraction. You learn new things from them, about them, and with them. These are all things of growth. If you are no longer excited by their presence, you are no longer learning new things with them, then you've begun dying with them.