Is it Okay to be Sad?
Solomon said it was.  In Ecclesiastes 3:4 he said that there is an appointed season to weep and a season to mourn. 
Later in chapter seven he wrote that it was better to go to a house of mourning than to a house of feasting.  His reasoning was that it would put us in mind of our own mortality and we would take it to heart.  Solomon then said that sorrow is better than laughter 
because when a face is sad a heart may be happy.  What does that mean?
I believe Solomon is telling us that it is worth taking the time to grieve over a loss.   Unless we grieve, and in the process of grieving deal with the fact that we too will someday die, we cannot properly put it “behind us” and move on effectively to the other aspects of life.
Part of what makes this true is that grief generates emotional energy.  This energy must be expressed, or it will create turmoil within us.  Only by burning this energy in our grief do we clear the way for joy.
Perhaps a good analogy is that of a flat tire.  No one enjoys dealing with a flat.  We may be in a hurry or may simply not know how to change a tire.  Regardless of what we may want to do, there is really no choice but to stop and take care of the flat even if we have to get help to do it.  Only then will we be able to “get back on the road” properly.
Jesus cried over things that gave him pain on more than one occasion.  He taught us that those who mourned were blessed, Matthew 5:4.  We should pay close attention to the words of our Lord and His servant, Solomon.  I believe they would both agree with this piece of late verse:
        I walked a mile with pleasure, she chatted all the way,
             Yet she left me none the wiser for all she had to say.
        I walked a mile with sorrow, and n’er a word said she,
             But oh, the things I learned, when sorrow walked with me.
Joy is a fruit of the Spirit of God (Galatians 5:22-23) while sadness is not mentioned as such.  Nevertheless, a seed must die to become the seedling that becomes the sapling that becomes the tree that bears the fruit.  In other words, embrace sadness when it is “in season,” just don’t let it rule your life.