"Ignorance is bliss" is a very old adage that often applies.
People with high I.Q.s and those who are very analytical thinkers are likely to find it difficult to "live in the now".
They're always trying to prepare for what might go wrong. Their biggest fear in life is being "caught off guard".
Another facet is they often come to see (birth, life, and death) as a meaningless game which always ends the same. You die!
Why continue to live? (They may want off of the merry-go-round)
They become cynical at times when they look at people who are laughing, playing, and enjoying life. "They're just too stupid or blind to realize how bad things are in this world!" they think to themselves.
In fact some people go out of their way to avoid being around "happy and positive people" because they seem too optimistic about life.
Whenever someone believes (they) have it all "figured out" there is little joy and surprise left in life for them. Most children will tell you they were happier when they believed in Santa Claus as opposed to when they learned he didn't exist.
Knowledge has a way of eliminating extreme highs and optimism. If one never learns to live in the moment they'll never feel true joy.
Sometimes it's best to focus on being happy right (now).
"Worry does not empty tomorrow of its sorrow. It empties today of its strength." - Corrie Ten Boom