A word I use far too often, only because it so closely resembles, both in sound and meaning, the impression I'm trying to communicate. There are a grave shortage of words describing praise or excitement in the first place; we all overuse beautiful, wonderful, great, gorgeous, awesome, and fantastic, and like the boy who cried wolf, they become less powerful each time.
But the word "sweet" stands out in my mind. It has its low moments, like the times when a girl will tell a boy he's sweet as a way of saying she's not interested in a spineless pushover like him. But as a word to describe those innocent, happy, serendipitous moments that make you uncommonly and irrationally contented, a better word can hardly be found.
I go through crushes like a Suburban goes through gasoline, and "sweet" is the only word that comes close to describing what these crushes are like. I honestly have no thoughts below the waist, I'm just terribly happy to see them, and I smile when they smile.
I'm often in a sing-song mood for no apparent reason; sometimes I even skip around, oblivious to what a dork I must look like. I could not accurately describe my attitude toward life at moments like these without using the word "sweet."
And it goes without saying that looking into the eyes of the one you love is perhaps the kind of moment that the word was created to describe.