Love can flourish only as long as it is free and spontaneous; it tends to be killed by the thought of duty. To say that it is your duty to love so-and-so is the surest way to cause you to hate him of her.
There is an old saying that, you can't kill a frog by dropping him into hot water. As you drop him into the hot water, he reacts so quickly that he immediately jumps out unharmed. But if you put him in cold water and gradually warm it up until it is scalding hot, you have him cooked before he knows it. The encroachment of bad habits in our lives is very much like this.