We do not have to be happy with everything that happens each day, but we should know this: Even though we may not be able to change or prevent these events, we can change our attitude and reaction to them.
With such amazing beauty all around us at any moment, that is further justification of why we should focus and be entirely in the now, with no distracting thoughts to steal the thunder from the simple wonders all around us.
Take comfort in knowing that things that seem opposite can, and must, co-exist as complementary pieces in the same larger object. Two sides of a coin must exist for the coin to be a coin.
With that said, it is also not correct to think that people cannot change their behaviors or personality. The mind is very powerful, and with humans having such a powerful conscious and conscience, we can change our perceptions and drastically alter our behaviors and habits.
We may want to spend more time with those we love and realize what is truly important in this life while we are here. That is the part that is not pessimistic or optimistic. It just is, and knowing the impermanence of everything should help us learn to cherish every moment we have with everything. This is the beauty of truth.