An age old question, and still a very interesting one.
I've seen movies abotu a utopian future in which people are more level-headed and even mooded, there is no real pain, but then again, there is no real euphoria and high joy.
Without bad, good has no meaning.
Having said that, I see no reason for there not to be a majority of goodness and calm happiness in our lives, but still with great and joyous times too.
The main source for the ups and down, I feel, is human relations, and more specifically - love.
Love is the source of the greatest joy and strength, and with that - also the biggest weakness and despair can come of it.
'Better to have loved and lost than never loved at all' is an interesting phrase, which I feel is related to this.
We love people, and we lose them. It's natural to fear loving, as an investment of emotion, time, and feeling that can all be lost.
Are memories a blessing or a curse? I think love and time are the most important gifts we possess, and we must use them wisely, and dont' always fear to regret, as the biggest regret many people have is that they feared regretting too much

'Do You Realize - that everyone you know someday will die '