I've thought about that question myself for a while now... I can only think of two distinct situations where I was overcome by happiness to the point that I'd call it the best of my life, both are rather odd, and seemed to be something created in my mind as a reaction to something that would normally be seen as a casual situation. First, was watching an episode of the anime FLCL, throughout the whole thing, I felt this extremely surreal presence, it was fantastic, and I have no clue why... it was simply euphoria. Another time was watching the movie "But I'm A Cheerleader" which caused a similar bout of just pure happiness, for no reason that I could point out. It may seem rather pathetic that my happiest moments were caused by watching television, but not even I can explain to you why I had that feeling towards them, but it certainly was great. Can anyone relate?!