All the external phenomena exhibit three characteristics:-
3. Not- Self
It is a human psychology to search pleasure in the external phenomena, which have a certain nature of impermanence. Some thing which is in my possession right now may not remain with me the comming moment.
I purchased a valuable wrist watch. When it was in my possession it belong to me. Next day when I went for a morning walk, suddenly a man appeared on a motor cycle, snatched the watch, and fled away. I was pleased when I bought the watch. The incident of snatching mad me sad. This was only due to my attachment.
In another situation my friend also bought the same brand wrist watch. The next day his watch too was snatched away.
The first instance of buying of watch by my friend may not give me pleasure. The second incidence of snatching away of the watch do no disturb me too.
Like wise our body also have the nature of impermanence. We take birth followed by sickness, old age and death. In the same way our mental feelings and perceptions too are impermanent.
By ignorance we establish our attachment to these impernanent external phemona, which is the root cause of unsatisfactoriness.
While meditating we contemplate this nature of impermanence and refrain from developing either craving or aversion.