Every event causes some pain.Pleasant or unpleasant.
If you look into the pleasure or joy that you get in life from the time of birth, they all come with a tax. You have to pay a tax for that and this tax is sorrow. In the next sutra, Patanjali says:
“Parinama tapa samskara dukhaihi guna vrutti virodhachcha dukhameva sarvam vivekinaha’’ (II Sutra 15)
Life is Beautiful 