Why do some people protest an Election?

Even at my age, I do not understand why certain groups of people "take to the street" when something does not go their way.  If someone can explain it to me please do. 

Here is an example:  Why are people protesting now after the election, but did not protest after the election 4 or 8 years ago?

I was disappointed 4 and 8 years ago but I did not hit the street. 

Please someone explain it to me!