Well i think love usually grows from habbit.What starts out as hormones raging then turns into a deeper affection from time spent with that person.I mean you might fancy the arse off someone but they turn out to be a right dickhead
The love for your parents came from habbit as they looked after you from birth.
I think the only one exeption would be the love for your children since you have already established a deep bond with the other parent and your children are an extension of you and the person you have created that bond with.
Love is natures way of protecting the young.Not all animals stay with their young but the more advanced species do.