LOL it doesn't sound like tis possible does it? Well it's basically when you spend all day doing something SO awesome (usually with someone you have feelings for) that the next day when you go back to your normal life, you feel sad that every day can't be like that amazing day. The reason it's happy is because nothing "bad" happened. Something good happened that you wish would happen more often. It wears off within a few days or so, and is nothing that will last. What will last is your increased feelings toward the your "crush" so to speak.
Guy: Dude, how was yesterday with (insert lover's name here)?
Other Guy: It was amazing! I wish that everyday could be as amazing as yesterday. I've got bad case of Happy Depression.
Guy: Sucks man
Other Guy: IKR