A given person likes (loves) you as much as you like (love) him or her.The idea is just meant to be provocative, and he urges doubtful readers to "keep it in the back of your mind, and see if it proves useful over the next few years."
I find the symmetry thesis a surprisingly strong predictor of human behavior and inclination.
Do I want to know how much you like me? It is simple. I imagine how much I like you. (If you do the same, are we circular? Or does some kind of fixed point theorem apply?)
Unilateral crushes are possible and indeed common, although with repeated contact they usually collapse into symmetry, one way or the other.
I can imagine several (non-exclusive) mechanisms in support of the symmetry thesis. Perhaps "having a connection" -- which is mutual by nature -- is the key to true liking and attraction.
I think the amount X feels she could love Y needs to be differentiated from the amount X does love Y. I imagine that each successful romantic relationship (that is, pair of people who get at least as far as dating happily) is preceded by several failed connections in which the desire was great but the reciprocity not. To move on, I think we tend to remember these as more even than they really were; I am always surprised to see the intensity of emotion in my old journals towards people and situations that now evoke only a glimmer of what they once made me feel.
One commentor writes:
I think a common theoretical arguement against the symmetry thesis is the "Groucho Marx Syndrome", made manifest in Woodie Allen's Annie Hall: 'I refuse to join any club that will have me as a member.'Different types of people, like Alfie and Annie, may have a tendency to love/like people who don't reciprocate evenly--who either show them more love or less.
This maps into the truism 'wanting is better than having,' which has moderate support from hedonic theory.
Of course, after Alfie rejects Annie, he regrets his decision and his love returns.