Love within this context is based on a sense of loss. People come together not based on what they are but what they can give to each other. This understanding of love may be backed up by common sense. In many families, one parent attends to child-rearing and other at-home tasks while the other brings in an income. In such cases, each parent brings something else, and through this interaction, their family unit is stronger. Plato, a philosopher in ancient Athens, wrote in the symposium something similar. He believed that each person only had half a soul and by finding one's other half we became a full person. It is likely that the Spanish dictionary took inspiration from this ancient philosopher. Many of us identify love this way but don't recognize it. We all know the saying "opposites attract"- we fall in love with those different than us. People are fascinated and enthralled by the unknown, and we as humans are inherently curious. Our curiosity drives us to love those who are different than us and who have that which we don't because we are curious to know something different from ourselves intimately. Love is formed because people need some sort of companionship to thrive and have a more functional lifestyle.
One does not need to lack something to be in love. Plenty of people have everything they need and want, yet still fall in love. In fact, many believe one has to be happy with themselves before they can successfully love another, which directly contradicts the idea of the lack of something being necessary for love. Additionally, a large portion of people only fall in love with those they are similar to- shared interests and beliefs lead to compatibility, so a lover isn't filling in something one lacks, but accentuating what one already has.
[P1] We love that which is different than us. [P2] Love gives most people a feeling of completeness or wholeness. [P3] Love is the filling of our insufficiency.
Rejecting the premises
[Rejecting P1] Many people love what is the same as them.