Bhagavad Gita Verse 35, Chapter 18

yayaa svapnam bhayam shokam vishaadam madameva cha |
na vimunchati durmedhaa dhritihi saa taamasee mataa || 35 ||
 
That by which one with an inferior intellect does not give up sleep, fear, sorrow, dismay and intoxication, that fortitude is considered taamasic.
 
yayaa : by which
svapnam : sleep
bhayam : fear
shokam : sorrow
vishaadam : dismay
madam : intoxication
eva : also
cha : and
na : not
vimunchati : one gives up
durmedhaa : inferior intellect
dhritihi : fortitude
saa : that
taamasee : taamasic
mataa : considered
 
Fortitude is the quality of the intellect by which it holds on to certain thoughts, and rejects others. From the start of a task to its end, our intellect is confronted with a barrage of thoughts. If it drops the thought of completing the task and starts holding on to other tasks, it will never be able to take the task to its conclusion. Sattvic fortitude is active for all tasks, raajasic fortitude is active only for tasks motivated by selfishness and sense enjoyment. The third kind of dhriti or fortitude, one which holds on to everything but the task at hand, is taamasic.
 
While performing any task, it is natural to expect obstacles, and some fear as well. There are moments when our body and mind is tired, and we need to take some rest. If some things do not go as planned, we feel sorrow. If things do not go as planned repeatedly, we may also feel depressed or dejected. Conversely, if we experience some temporary successes, we may get intoxicated with the mental high, with the pride generated by those successes. Shri Krishna says that the fortitude that drops the thought of completing the task and holds on to any of these temporary thoughts is taamasic.
 
What causes someone to develop such a negative kind of fortitude? It is the degree of discrimination inherent in the intellect. One with a moderate level of discrimination has raajasic fortitude. But one in which there is little to no discrimination has no clue that he is choosing to hold on to thoughts that will ultimately lead him to ruin. Or to put it another way, one who has perversely high level of attachment to a certain object, person or situation will lose whatever level of discrimination he has left, due to the pursuit of that object of desire.