Are people happier today than they were in the past?
I don't know if we are happier, but we have more opportunities for happiness than we did in times gone by. Most Western people have the privilege of living in much better conditions: we have good health, education, social security, etc. That material stability means we're free to pursue pleasure through leisure and we have more time to seek happiness. But some people are still better at it than others! There are social, material and psychological inequalities when it comes to happiness and aptitude, but you can "learn" it and you can work on your aptitude.
Why is the search for happiness taken so seriously?
It's a neverending quest. Ever since Ancient Greek and Roman times, philosophers have been studying eudaimonism, or the pursuit of happiness. It's a human thing: we're the only animals who know we will eventually die, and that's really destabilizing and disturbing for us. Happiness is like a moment of immortality, when time stops. It's the only antidote to the fundamental sad truth of human existence.