What is it about a person do we truly love?
Is it the look?
Think back to the first time you saw your significant other. Did you say "WOW" in your mind? If so, how long did it last till you start seeing he/she like anyone else. Off course the one you love is different, but the looks eventually become something more or less an aspect of relativity rather than the soul reason why you love them. If it is then you are in trouble because eventually we all get old and unattractive.
Is it the personality?
Well if it is not the looks, hopefully, then it must be the personality that creates attraction. This notion seems rather intuitive, but is it? So think back to the last time you had a conversation with someone that you felt was truly compatible. How did it turn out? Lets assume that it turned out well and a relationship fostered, how long did it take before the conversation that used to last hours now only last minutes. How long did it take before it starts becoming casual rather than those soul-searching talks. Eventually, you start feeling like it is no different than any other ordinary conversation you may of had with other people. And soon you will start to realize personality does change.
The bottom line is, it only seem so perfect the first time because you made it that way. You created an environment where you had hope that this person will be the one. Maybe you were sad, and they said everything that you wanted to hear because you were feeling bad and it was you who declare it to be perfect by your own definition.
Either way, it seems like trial and error. You can't love someone for a single reason. Love, rather, is a compilation of all the different characters that makes up you and I. Since the variation is so huge, we are able to create very unique ones that only the parties involved can appreciate. A relationship will only work if there is compromise, trust, forgiveness, unselfishness, openness, willing to accept, willing to adapt, goal oriented and driven to pursue a lifetime of happiness together. Without the whole package, individual reasons like looks and personality alone will eventually die out. Besides, can we say that we like everything about our significant other? No body does, but love will drive us to make it work because if one component falters, the others will make up for it.
So what is it about a person do we truly love? The answer is nothing. We do not love them for merely being them. We love them because they have giving the things (compromise, trust, forgiveness, unselfishness, openness, willing to accept, willing to adapt, goal oriented and driven to pursue a lifetime of happiness together) we needed to make us complete. To make us happy. We love the interaction, memories, engagement. We invested and it is the product of the relationship, whether children or good time, that is what we truly love.
This is the reason why relationships go sour. When one party decides to stop giving, it is then that the relationship comes to an end. It is not because one person stops being beautiful or that their personality has changed so much that we can't stand them. It is simply that they have stop giving and the process manifest itself as a personality change.