"A tale of two cities" by Charles Dickens.

Since the beginning of time, it has been impossible for humans to love unconditionally. Even God, the supreme being of the human race, cannot be loved unconditionally by His people unless they totally give themselves to Him and worship Him -- in other words, unless they sacrifice themselves. Considering this, it has been hard for humans to even try to love one another without selfish motives creeping into their unconscious. Likewise, parents and their children …

…conditions, including love. Would a husband love a wife if she were dishonest and unfaithful, knowing full well that his wife would cheat on him for many more times to come? This is not likely. Would a child love its pet if the pet bit the child one day and the child knew this would happen again? Again, probably not. Thus, it is unrealistic to expect the love between child and parent to be unconditional.