According to the preface, why did Mary Shelly write Frankenstein?

From the text:

It was commenced partly as a source of amusement, and partly as an expedient for exercising any untried resources of mind. Other motives were mingled with these as the work proceeded. I am by no means indifferent to the manner in which whatever moral tendencies exists in the sentiments or characters it contains shall affect the reader; yet my chief concern in this respect has been limited to the avoiding the enervating effects of the novels of the present day and to the exhibition of the amiableness of domestic affection, and the excellence of universal virtue. The opinions which naturally spring from the character situation of the hero are by no means to be conceived as existing always in my own conviction; nor is any inference justly to be drawn from the following pages as prejudicing any philosophical doctrine of whatever kind.