Yoga Sutra 2:7 and 2:8

Sutra 2:7
Sukha anusayi ragah सुखानुशयी राग:

Pleasure is accompanied by attachment

Sutra 2:8
Dukha anusayi dvesah दुखानुशायी द्वेष:

Sorrow is accompanied by aversion

This two stutras go hand in hand, attachment and aversion are two root causes of happiness and sorrow. Shri Kirshna has explained this very well in Bhagvad Gita (post on this later). We are happy when we get the things we want, an outcome we want when we don't get it it causes sadness and pain. We need to bring our minds (by meditation) to a state where it is neither attached nor averse. This doesn't mean a yogi doesn't feel happy or sad, she just doesn't get overwhelmed with the happiness or sadness. She is stithapragya, when she gets something she doesn't      get overly elated and when she doesn't get it she isn't sad as well. You can only create this state of mind by knowing Self (brhaman - ब्रह्मा or  atman आत्मन), realizing that you're that Self and not different from the Self. You're complete the way you are (more on my experience on this practice in the next post). This knowledge has to be constantly with you and that comes from meditation. Meditation practice connects you with that awareness.

Here's what B. K. S. Iyengar has to say on these two sutras:

2:7 
Dwelling on pleasurable experiences ignites desire and a sense of attraction, which creates attachment. Pleasurable experiences generate greed and lust, which strengthens attachment and stimulates a greater craving, as one always wants more and more. One becomes absorbed by the pursuit of pleasure, and addicted to gratification of the senses.


2:8
Pain, sorrow and misery triggers a chain of hate or aversion. Recollecting lost pleasures, tormented by desires unfulfilled, man is led to sorrow. 
A discriminating person strives to acquire knowledge so that he may strike a balance between sukha and dukha and live at the mercy of neither pleasure nor pain.


B. K. S. Iyengar has used the word discriminating person here for stithapragya.