Sorrow and joy go hand in hand. Both are equally important in life and we cannot avoid any of them. Without sorrow there cannot be happiness. As if there will be no sorrow in our life then we would not be worried about anything. Sorrow helps us to grow, without sorrow we would remain at the same level and would never try to go higher.
For example, if I draw something and post it somewhere or show it to someone, then if the other person points out some error in it then at first I would feel bad. But then I will try to overcome that error and improve myself. And once I have overcame the error, I would be happy as I would have learnt something new. But if no one says anything, even after spotting the mistake, then I would never be able to find error and would never be able to improve it. Hence, I will always remain at the same level.