There is only sorrow in this world and not happiness. Therefore sorrow only is felt. Desire begets sorrow and sorrow happiness.
दुःखमेवास्ति न सुखं तस्मात् तदुपलभ्यते। तृष्णार्तिप्रभवं दुःखं दुःखार्तिप्रभवं सुखम्॥