Why is it that the parents, specially the mother, generally have this unmatched love for their children?
What is the psychological reason behind it?
Is it more biological than psychological?
Attachment theory in adults is concerned with care giving ability toward ones friends, family, lover and child. Oxytocin is the bonding hormone. If you have enough of it in theory you will be a monogamous, child loving adult. If you want to con someone by a grift you would secretly give them Oxytocin because it induces general trust.
However it is a misnomer to believe that the relationships formed between parent and child are always lifelong and strong. To examine the psychology as apposed to biology of what makes the feelings between a mother loving for a child lets look at what makes feelings for a child loving toward their mother.
To join this wonderful planet earth as a human being we have to spend around 9 months in the uterus of a woman. During this time she feeds us and her neurochemicals train our brain unconsciously. If she is stress there is a higher chance the we will be deformations. Our predominant auditory stimuli is her voice and the choice of company she keeps. After we survive maturation in the womb her breast sustain us through Colostrum and breast milk. They provide hormones like oxytocin which make us like her, advanced antibodies which keep us alive while our immune system develops and water, protein, sugar and fat which nurtures us and quenches our thirst. This is our first positive association.
Not all mothers want their children. The first psychological indicator of your abilities to easily form a loving relationship with your children is how your mother treated you the first three years. If your mother did not properly form an attachment to you then you will be at a high risk for an attachment disorder. A state where its very difficult to have social experience and behavior matching social norms. Therapies for attachment issues are controversial and in many cases flat out bad. However not all adults with attachment disorders are bad parents, being abused doesn't force you to be abusive. It does mean you may lack skills learned in childhood which others can inversely apply.
Even high functioning very ill nonviolent (or controlled violent) sociopaths (ASPD) can make fine parents if they work hard enough at it.