I have always wondered, what exactly happens when you are into a relationship. One of the first things that comes to mind is preoccupation. Your mind is heavily preoccupied with the concept of sharing everything (everything that you are comfortable sharing). So I guess in the neurology of it, probably the neurons containing the image of this person in your mind (what you think the person is, will be, your general beliefs about where it can go, how far/deep etc.) try to link with all the neurons containing your daily thoughts, beliefs and activities. Its kind of like a dam being opened and the water from the 2 sides mix into each other. Its literally the merging of your individual identity with another person. I guess the reason its so pleasurable is because of a fundamental longing for a oneness experience. This longing seems to be the deepest longing in the human race but it might manifest itself in a multitude of forms we see today such as friendship, sex, family, other relations etc. We define identity by the memory of experiences and concepts formed out of that. So identity seems to be maintained by different kinds of memories alone. By this logic, our whole identity is derived from our past. I would presume our capacity for future projections too is heavily influenced by our past too. Merging identity is making you larger than yourself. You have a brand new set of things to explore, kind of like a child seeing a new toy for the first time which may explain why there is this initial burst of curiosity and attraction which fades slowly as familiarity increases and converts itself to a general attachment. I guess the root of this attachment is because we are attached to our identity in the first place. So when we merge someone else with our definition of self, we get attached to this person as much as we were to ourselves.
But in the field of spirituality, Ive noticed that there is a completely different take on this matter. They debunk the notion that identity is real in the first place. Also, if identity is an illusory thing then there is no need to be attached to it in the first place. This is a completely radical line of thought since if you are not attached to your identity itself, then you would not get attached to anything for that matter including relationships, experiences etc. Once you remove all attachments, what remains? Regarding there is a unanimous agreement that what remains in unconditional love and compassion.
I sometimes feel we are greatly restricting ourselves by loving only a few special people in our lives. I guess the reason for this is attachment to our own identity and only the people who conform with that are loved by us. We may call them different names such as family, friends, romantic partners etc. but they are all just labels pointing towards a single experience.