"Lithium salts have been shown to be very effective in controlling the effects of manic depressive disease, and you've probably at some time in your life felt a special 'chemistry' for another person. Studies suggest there is literally chemistry going on between two people who are attracted to each other. 'Falling in love' apparently causes changes in the chemistry of the brain; chemicals are produced that give that 'high' associated with a new relationship. Unfortunately, chese chemical effects seem to wear off over time, even if the relationship persists and grows."
edit: I'd post the "What did you drink last night?" thing, but I'm having a heck of a time getting the right person to show up. ;)