From amber love pj:
In this country, every person has to be matched up with a partner. Partnerships are made around when you turn three or four. The doctors decide who gets paired together. One person acts as the 'supplier.' The other as the 'receiver.' It is impossible for a 'receiver' to continue to live without periodically being given power from the 'supplier.' And in the same way, it is imperative for a 'supplier' that a 'receiver' accept some of the power generated within their body. This 'power' can't be made anywhere other than inside a 'supplier's own body… So for the 'receivers,' we 'suppliers' are completely indispensable....

Note: The doujinshi for this series have now (as of April 2009) been collected into an officially published volume (volume 3).