Since there are so many desires that arise, some get fulfilled, and some don`t. If you remain happy whether the desire is fulfilled or not, then it does not become a thorn.
For example, suppose you want to watch a movie and you go to the cinema hall, but there are no tickets available,will you get upset? If you get upset then that desire has gone into your heart. If it has not gone into your heart, you will say, 'Oh, I didnt`get the ticket, never mind. I’ll watch another movie, or go for shopping, or do some sightseeing'.
There are two types of desires:
1. The desire of the universe, by which everything runs. It is an instinct
2. The desire that you take into your heart and nurture
I am not classifying desires as good or bad. However, there are some superficial desires which come and go that do not achieve anything great.
When a desire gets into your heart, that can make some big thing happen. Mahatma Gandhi had the desire to free the country. That desire got into his heart and when that did not get fulfilled, he got upset.
Then, what did he do? He went into silence, prayer and fasting. He nurtured that desire in a very different way. He did not get cynical, sad, or frustrated because he had realized that it was a desire in his heart and so you would never find frustration in the face of that old man. The same with Nelson Mandela. So that desire did not turn into a thorn.