Bhagavad Gita Verse 23, Chapter 5

shoknotihaiva yaha sodhum praakshareeravimokshanaat |
kaamakrodhaadbhavam vegam sa yuktaha sa sukhee naraha || 23 ||
 
He who, even before this body departs, is capable of withstanding the impulse born out of desire and anger; that person is a yogi, that person is blissful.
 
shoknoti : capable of
iha : this body
eva : even
yaha : he who
sodhum : can withstand
praak : before
shareera : body
vimokshanaat : departs
kaamakrodhaadbhavam : born out of desire and anger
vegam : impulse
sa : that is
yuktaha : is a yogi
sa : that is
sukhee : blissful
naraha : person
 
In this shloka, Shri Krishna points out exactly why does the mind rush out into the material world. He says that in a person who working towards purifying his mind, there still exists two major obstacles to realization: desire and anger. Both of these generate impulses that cause the mind to go out into the material world. Even though the seeker cannot get rid of desire and anger, but can check the impulses caused by them, that person will attain happiness from within than from without.
 
Let's examine what exactly is desire and anger in the framework of the Gita. Whenever one encounters sense pleasures, or recalls a memory of a sense pleasure, the need to re-experience that sense pleasure is called desire. Conversely, whenever one encounters a sorrowful circumstance, or recalls a sorrowful memory, the repulsion generated is called anger. Both these emotions generate impulses to chase after, or run away from an object, person or circumstance. For the mind to realize the eternal essence, it needs to be even-keel. But these impulses take the mind outwards and destabilize it.
 
Now, there is a possibility of a Catch-22 situation here. There will be an impulse within us to seek joy. Unless we redirect it inward, it will always rush outward. But if we redirect it inward and it does not find joy, it will again go outward. Therefore, turning this impulse inward has to be done intelligently. If all we do is cut ourselves off from the objects that give us pleasure, it will not work. Instead, we must work diligently towards purification of our mind and performance of svaadhyaava, whether it is reading of scriptures, or pooja and so on. As our mind finds exponentially more joy in svaadhyaaya, it will automatically turn inwards and drop its fascination with external objects.
 
The next two shlokas describe the state of the person who finds inner bliss.