Could be a sign of mild hypomania. Along with this, do you also not need to sleep a lot, have rapidly changing mood swings, feel "on-top-of-the-world", feel more talkative, and/or more energetic? I've never been diagnosed with a mood disorder, but several doctors have told me that I could be mildly hypomanic, and quite frankly, I wouldn't be surprised. I have this laughing thing, too. Every little random thing will make me laugh silently to myself and smile, and I can't stop. I probably am cyclothymic, which is a VERY mild form of bipolar disorder b/c I've never needed sleep (went on two hours a few nights ago), I always talk fast, and I'm very moody. This could be a mild form of hypomania, especially since you're in your late teens (peak onset) and you've had depression in the past. If you have any questions, feel free to ask. God bless!
-GatsbyLuvr1920-