This is a quote from the Screwtape Letters, Lewis's provocative, creative critique on the Christian life from the perspective of the demons. So the speaker is the demon Screwtape, and "the Enemy" is God. 

"The Enemy’s demand on humans takes the form of a dilemma; either complete abstinence or unmitigated monogamy. Ever since our Father’s first great victory, we have rendered the former very difficult to them. The latter, for the last few centuries, we have been closing up as a way of escape. We have done this through the poets and novelists by persuading the humans that a curious, and usually shortlived, experience which they call ‘being in love’ is the only respectable ground for marriage; that marriage can, and ought to, render this excitement permanent; and that a marriage which does not do so is no longer binding.

"This idea is our parody of an idea that came from the Enemy. The whole philosophy of Hell rests on recognition of the axiom that one thing is not another thing, and, specially, that one self is not another self. My good is my good and your good is yours. What one gains another loses. Even an inanimate object is what it is by excluding all other objects from the space it occupies; if it expands, it does so by thrusting other objects aside or by absorbing them. A self does the same. With beasts the absorption takes the form of eating; for us, it means the sucking of will and freedom out of a weaker self into astronger. ‘To be’ means ‘to be in competition’."