Happiness&Sadness

Why is it that happiness and sadness are so closely alike? There are hardly any differences between the two, and so many similarities.
The sound of crying and the sound of laughter could easily be mistaken for the other.. But why? Could it be that the two are practically the same thing? Being sad, or crying can sometimes make you feel good or happy, meanwhile, being happy can sometimes make you feel sad, or guilty.
When you're very sad, years may leak from your eyes. And when you're very happy, tears might also leak from your eyes. So could these seemingly completely different emotions be one? I believe that one cannot go through life with only one of those. Or at least, a balance between the two.
Even those with lives that seem to be so great, and happy feel sadness, no matter how petty or foolish it may seem to another. All the same, people with seemingly tragic and sad lives do feel some happiness. Even if its by the simplest thing, like a blade of grass swaying in the wind.
I don't think anyone should fret, because if you are going through a sad time, happiness is sure to follow it.
Its like peanut butter and jelly... Or love and marriage.
Its impossible to have one without the other!
They go together like a horse and carriage..
Black carriage, white horse?
Basically, don't worry, because although you may be kept or surrounded by darkness, happiness is still leading you forward.