There was a thread linked to in at least one post previously, regarding the limited 'euphoric' stage of being in love. According to the article, the chemicals produced which cause people to be irrational and obsessed while in the early stage of a relationship have a time limit, and are then replaced by other chemicals which are more stable and 'settled', prompting the next stage in the relationship.
Does anyone remember what this thread was called, or which other thread it was later linked to in?