So how can anyone be certain about anything? The answer is simple. You can't be certain. When you encounter someone that tells you they know the answer to life's questions, absolutely, for certain RUN! They're taking the lazy path. They're not certain they have just stopped asking questions and are now expecting you to do likewise.
Look at these responses, quoted from some of the world's great thinkers regarding the issue of certainty:
"Certainty? In this world nothing is certain but death and taxes." - Benjamin Franklin
"The quest for certainty blocks the search for meaning. Uncertainty is the very condition to impel man to unfold his powers." - Erich Fromm
"Doubt is not a pleasant condition, but certainty is absurd."