To be the thing or person that ever happen to your romantic partner, then you would need to become many things and persons for them (the big 4 will be talked about below). You just don’t want to be a figure head person with inefficient title of a lover. Your goal is to be the most valuable person in their life, desiring to become the most important person in the life of your partner is not a selfish thing to do. By all standard, you are the most qualified and privileged person to become so, failing to do so when you actually can means someone somewhere will take the position away from you and you would be forced to play catch up with that person in the life of your partner.
You wouldn’t want to be in this situation because you will be frustrated living a tenant in the heart of your partner. Also, it deprives you of some stars and magic to be experienced in your relationship.