Vaisesika Sūtra 10.1.01

iṣṭāniṣṭa-kāraṇa-viśeṣāt virodhāt ca mithaḥ sukha-duḥkhayoḥ arthāntara-bhāvaḥ ॥1॥

Due to distinct causes of desirable and undesirable and their opposition, pleasure and pain are of a different nature.