Of these, sattva guṇa by dint of its immaculate nature, is the cause of light (knowledge) and harmony. But, O sinless Arjuna, it binds one by causing saṅga (clinging) to happiness and knowledge.
Each guṇa has its distinct property. In fact, only through its property, we can identify the guṇa. Krishna emphasizes that despite the distinctness of each, all guṇas bind the mind and intelligence. This bondage is spiritual, not physical.
Sattva-guṇa is the cause of light and harmony, but it causes a strong addiction to happiness and knowledge. Through that the sattva-guṇa creates bondage for the embodied soul. Any clinging or attachment is an obstacle in the path of spiritual perfection and freedom. All experiences ultimately involve the mind and intelligence. They can lead to a variety of emotions. The one identified as immaculate happiness is rooted in sattva-guṇa. In fact, we can discern sattva-guṇa only through sāttvika happiness (18.37) and sāttvika knowledge (18.20) it results in.