Its true,to realise what true happiness is you have to experience sadness in your life.It might not be too difficult to imagine the truth in this fact but the complex emotions of a human being are very interesting and difficult to understand.
A great example of this fact is when a person who has got used to his society and lives in a routine manner can sometimes take his friends or family for granted.He may never truly appreciate their emotions not because he is not happy with them or because he does not love them but because he does not truly understand the happiness they give him.When this person loses a friend then in the remorse he feels due to their loss he begins to truly understand how happy he was with his friend.
So what is exactly happiness? I don't think there is an exact definition for happiness.It might be monetary success for some or a loved one for others.What is sadness then? We cannot define it exactly too. It is generally associated with pain or suffering which in the end means lack of happiness.
So happiness and sadness are like Ying and Yang.One cannot exist without counterbalancing the other.So we can basically define happiness as the lack of sadness.It might not be an exact definition but it does definitely explain it.
Hence,people who lead a routine life in this busy world should realise that they have to appreciate the fact that they are happy and they can only do this once they know what sadness is.
So in my view you do need to know what sadness is for you, to really be happy.