So that's how the relationship begins in the first place. You lack something within, and to make up for it you're going to the other person; you're looking at the other person primarily in a utilitarian way.
You want to use the other person.
Be it a man, be it a woman, be it any person, all of us feel a certain hollow within; there is a void, there is an emptiness, an incompletion that constantly bugs us. We feel it. We feel it, and it is for the sake of filling up that hollow that we work so much, that we do much of what we do, including striking relationships.
So that's how you get serious with someone.