Sadly, when you're not in a good mood, sometimes it's really hard to find someone to talk to. You're of course there to listen to everyone around you, but people don't want to listen to your problems. Slightly hypocritical, yes? And you kind of just want to spill your guts out to someone (and probably will at some random point on some unsuspecting stranger)? But no one's there. At least not that you can really talk to without feeling like you're burdening them more, or like you're whining. Of course even writing about it on something like, oh say a blog, is borderline whining, but you need an outlet. So as you continue to help Boy A with his relationship problems. And Girl A with hers. And Boy B with his self esteem issues and Girl B with her stress, your worries are pushed aside for later review. Of course you love helping the sad people in your life because at the end of the day seeing others happy makes you that much happier. It just becomes impossible to be fully happy when you're competing with others for your turn to vent, and you're ignored. You're only allowed 95% happiness and soon 90% happiness and then 85% and it decreases slowly until you can't hold it anymore (reference to the spontaneous bursting of tears on a random stranger from above).
People of the world, remember to check on daily those around you. EVERYONE wants to share some aspect of their life they believe to be "unsharable". Prove their theory wrong.