Watching the sunset

"The moments of happiness we enjoy take us by surprise. It is not that we seize them, but that they seize us."- Ashley Montagu

I've always struggled with the idea that you have to work at being happy. I understand the meaning but it just seems that that the work "should" only need to consist of pushing the things that don't make you happy out of the way. Not trivial work by any means but the focus is different.
Why shouldn't happy be the default state? It seems to be in children...And dogs. :)