"The secret of happiness is to see all the marvels of the world and never forget the drops of oil on the spoon." What he is trying to say is don't focus solely on one thing or another, you want to balance the task of the oil on the spoon but also look around and see what is around you. This can apply to a lot of different things in life. You don't want to go through life just worried about one thing, you have to go through life with a balance so you won't miss anything but also never to forget what also matters to you which is like the oil on the spoon.
People go through life just focused on one task and when they do this, they miss so many beautiful experiences life has to offer. When they miss these things life can become so boring.