I suspect that like all of natural selection that the better the bond between the nurturing mother and the infant the more likely the survival of the infant and thus those mutations that helped this bonding became part of the gene pool.

I don't think I can do better than you coberst. But I'll say what I feel on the subject.

Human love, in the eveolutionay sense of the term, I believe, is caused by the process of natural selection itself. Any and all forms of love imply a certain level of compatibility for love itself to grow and flourish.
Birds of a feather flock together, and the stronger and more compatible the flock of birds the higher the probability that the flock will survive. Compatibility means designed to work together. So if the flock is to survie they must be compatible.
In humans it is the same way. On a purely biological and psychological level we see what is truely in each other not to mention the exteriors. This is partly conscious, but, as it is with most everything else, it is mostly unconsicous. This accounts for the pardoning of superficial shortcomings in each other. A practice so highly embraced by the romantic sect of humanity (of wihich I am part). On a biological & psychological level at least, we offer each other more than we realize with the consicous decerning mind. Hence the fathers love for the prodigal son, the cinderella story, Beauty and the beast, the proverbial mothers love in general, etc.
The more that we have to offer each other in a purely evolutionary sense, the more we are willing to give each other. And the more we give and recive to survivals end the stronger the bond we share. "Platonic idea that the giving and receiving of knowledge, the active formation of another's character, or the more passive growth under another's guidance, is the truest and strongest foundation of love".
The unconsicous mind desigined for survival sake a mechanism called emotion. All of the sensations associated with our emotions are a direct result of neurochemistry. So in a love relationship, which as stated before comes from the transference of knowledge toward survivals end, strong and instinctual emotional sensations come in to play due to the nature of the relationship. It is the job of the flock to protect the flock. Here is where we find in evoloution the role and origin of love as we know it. And it is the contemplation and awarenes of our origins and ends, so well experessed in our unconscious by emotion, that creates the mystique in our conscious mind surroundoing love. Which I might add is the end and begining of all the religions (who created me? & where will i go?). Hence the spirituality of love. We need love like we need water. And frankly I find love more sattisfying. Because with out love there would be none to need water.

"Love is a temporary madness. It erupts like an earthquake and then subsides. And when it subsides you have to make a decision. You have to work out whether your roots have become so entwined together that it is inconceivable that you should ever part. Because this is what love is. Love is not breathlessness, it is not excitement, it is not the promulgation of promises of eternal passion. That is just being "in love" which any of us can convince ourselves we are.
Love itself is what is left over when being in love has burned away, and this is both an art and a fortunate accident. Your mother and I had it, we had roots that grew towards each other underground, and when all the pretty blossom had fallen from our branches we found that we were one tree and not two."
"Love is the beauty of the soul."
St. Augustine

Thanks again for the topic

I do not know exactly what I am talking about this is just theroy.

I hope not sophisty.

peace, harmony, and infinite love for the universal flock of man.

vajrasattva

Maslow defined the five needs of humans. The third level of this hierarchy of needs contains the matter of love. Wiki says:

Social needs
After physiological and safety needs are fulfilled, the third layer of human needs is social. This psychological aspect of Maslow's hierarchy involves emotionally-based relationships in general, such as:

• Friendship
  
• Intimacy
  
• Having a supportive and communicative family
  

Humans need to feel a sense of belonging and acceptance, whether it comes from a large social group, such as clubs, office culture, religious groups, professional organizations, sports teams, gangs ("Safety in numbers"), or small social connections (family members, intimate partners, mentors, close colleagues, confidants). They need to love and be loved (sexually and non-sexually) by others. In the absence of these elements, many people become susceptible to loneliness, social anxiety, and clinical depression. This need for belonging can often overcome the physiological and security needs, depending on the strength of the peer pressure; an anorexic, for example, may ignore the need to eat and the security of health for a feeling of control and belonging.