It had quite an impact on me, coz I'm this person who is kinda super-optimistic in life, and hence almost always happy. So when I heard this particular statement, it got me thinking. I wondered if I was 'useful' to anybody and not just happy without any meaning.
A universal statement: Happiness is contagious. You smile, and you create a wave of it. The effect is extremely satisfying. On the other hand, usefulness is not contagious, is it? My happiness cheers at least two people in a day. Thats the minimum number. A min. of two ppl would mention my super-(crazy acc. to a few)-optimism, and they would try being optimistic too. I think, ppl who are happy tend to be more productive-call it 'useful' if you want. If you're happy, you tend to be more creative.
Is it correct to generalize that all useful ppl are happy? I don't think so. All happy ppl are useful? Yea, a larger no. than the former. Then again, I don't know if I'm being biased to this happy-ppl-are-more-useful category just coz I belong to that category. What's your say?