Did you ever feel that " I want to DANCE today " even without reason ?
Today its happening to me 
Sometimes it happens that you don't now the reason for your happiness
or sometimes you even know something painful for you but from inside you feel some special joy or a special type of happiness and hence you either partially or completely ignore that pain.
I don't know that weather it happens to you or not but I know that it is happening to me.
Some reasons for this might be
- You knew some thing good for you but you are not supposed to show happiness for that
- You achieve some tiny target set in your mind but you were not supposed to talk about it.
- you get mental or physical relaxation after a long stress.
- Some thing good happen to you that you didn't notice or you think that you don't care about that
- or something really good is about to be happen to you very soon.
