The Biology of Love
Why do we need love? There are two main reasons - biology and psychology, and the two aren't separate, they are very much intertwined.
The moment you were born, at that point - at that moment of infancy, there was no concept of who you are. As an infant - at the time of birth, your brain didn't have a concept of "self." A child at birth cannot distinguish itself from the mother - there is no concept of self. When the mother holds and feeds the child, it feels familiar, safe and comforting.
Over time, as your brain and the nervous system developed, you formed a concept of who you are "special people" around you that keep you safe and happy - provide you with basic life and emotional needs. This affection, this care - from physical touch to food to safety and support, is what we start to translate into love and affection.
And this is what some of the first experiences that your brain ever had - your brain learned started to see "you" at the center of your world, and then "people you love" and the people who love you as your "immediate inner circle." Everyone else, is outside the circle.
In a way, this inner circle - your loved ones who provide this affection, care, and survival become an extension of you. And this is what you understand as love - this is what you understand as care, and that feels good - it feels a part of you.
Psychology of Love
Once your mind and body knows and understands what love is - this understanding is what your brain holds on to, even as an adult.
You want this love because your mind doesn't think of it as anything outside of you - your mind perceives love as an extension of you, just like it was with your first life experiences.
It is that intimacy that you seek as an adult - you're no longer a child, and with age and time, the parent-child relationship changes. What stays the same, is to be "that emotionally intimate," to be that "psychologically blended" with someone... to see them as someone other than you, and yet an extension of you... "your immediate inner circle."
And this is why your brain continues to seek love and provide love to feel and nurture that connection that seems above all other relationships and connections, and feels more intimate, more personal, more unique to you.
This becomes even more crucial as navigate the hard and cruel world, where you develop tougher and tougher exterior shells to get through your days, your work, and your life in general. That however, often leads to more isolation.