My understanding was that +/-'s to mood last as long as the thought does (a couple seasons IIRC), though that may be outdated. I think that is
Also, I noticed that there's mechanics in place to prevent any Dwarf from getting too happy: any Dwarf above 100 happiness gets their happiness decreased at season shifts.
Would account for a Dwarf losing more mood each season as they'd be experiencing a larger number of +s, which means the number they lose also increases. Do dwarf's have a default mood value? Never really checked for a just embarked dwarf, if they are even thought free at that point.
EDIT:From the wiki page on
thought, though its quite old.
A dwarf's emotional state is quantified in a single number. This number is computed by summing the modifier for each thought the dwarf is currently remembering. Each thought lasts one year, and after half a year has passed since gaining a thought, the modifier is halved, rounding away from zero (so 7 halved would be 4 and -9 halved would be -5). There is no way to tell the age of a thought.
All the values are listed as well for thoughts, though they must be outdated as most of the +/-'s are around 10-30 points, considering the heights my dwarves' moods are reaching (None of my dwarves currently in the top 3 for mood have an artifact related thought). Or the mechanics have changed a bit since the page was updated.