Bhagavad Gita Verse 20, Chapter 5

na prahrishyetapriyam praapya nodvijetpraapya chaapriyam |
sthirabuddhirasammoodho brahmavid brahmani sthitaa || 20 ||
 
Having obtained a desirable situation, he does not get elated; nor does he get perturbed if it is undesirable. The person of steady intellect is never deluded; the knower of the eternal essence is established in the eternal essence.
 
na : does not
prahrishyet : get elated
priyam : desirable
praapya : obtains
na : does not
udvijet : get perturbed
praapya : obtains
cha : and
apriyam : undesirable
sthirabuddhih : with steady intellect
asammoodhaha : without delusion
brahmavid : the knower of the eternal essence
brahmani : in the eternal essence
sthitaa : is established
 
In this shloka, Shri Krishna emphasizes the realized seeker's outlook towards situations in the material world. Like any other person, the realized seeker encounters pleasant and unpleasant situations. He cannot avoid them. But the difference is that these situations do not destabilize the realized seeker. In other words, they do not cause elation or depression.
 
Usually, when something undesirable happens to someone that we do not know, it does not affect us. But if it happens to us - say we fall sick - it affects us deeply. But the wise person has transcended the ego. He recognizes that every situation is part of nature. In nature, elements are interacting with the elements in multiple configurations. Once he has this vision, happenings of the world do not impact the realized seeker.
 
It is important to note that situations, both good and bad, have their place in our lives. Unpleasant experiences are useful from relative world perspective because they usually have a lesson embedded within them. With each unpleasant situation the emotion of sadness will arise. But in a realized seeker there will be a space between the emotion and the self. For example, if we are on a boat, the ups and downs of the sea will impact us. But if we are standing on a rock on the shore, the ups and downs of sea do not affect us.
 
Why do realized seekers remain unperturbed? Their understanding of the eternal essence has become stable and firm. They have become "sthitha prajnya". They are free from the delusion of doership and enjoyership. Initially, when this knowledge is first gained, it stays on the surface. We experience the eternal essence sporadically, for a brief instance, then immediately go back into identification with the body/mind/intellect.
 
So therefore, the one who constantly maintains the identification of the eternal essence becomes one with the eternal essence. He remains a pure witness of the ups and downs of the material world.
 
Footnotes
 
1. Tulsidas has said that the ocean remains same in rain or drought, but a small lake overflows and dries up. It is a simple yet powerful example of the different between a realized seeker's vision and an ordinary person's vision.