Because the sense of separate 'I' is essentially a mentally created illusion, it needs constant activity in order to continue existing in the mind. Like and dislike, attachment, aversion, jealousy, pride etc. are the main overt activities of the ego. The more of these mental activities/qualities we have the more alive we feel, the more the ego seems to be real and concrete. The bottom line is, the ego depends on desire and wanting, its' life blood is desire. The ego and desire are like the two sides of a coin--one cannot exist without the other. It is like riding a bike. As long as we go on pedalling the bicycle keeps moving foward. But if we stop pedalling the bicycle will start slowing down and eventually collapse. The more we go on generating desire the I/ ego seems very real. When desiring stops (as when in deep meditation) the ego begins dissolving and appears as an illusion. That is why it is almost impossible to give up desire and aversion. Giving up desire entirely is something like committing ego suicide.
This relationship of the desire and the ego is really the crux of the Buddha's Teaching, especially the 2nd Noble Truth and Anatta : The cause of Suffering is Craving; the cause of craving is Ignorance, the deepest level being the clinging to 'I', the conceit, 'I am'. This means not understanding Anatta.
Desire cannot be satisfied as long a person still ignorantly clings to the sense of 'I'. Actually the objects we desire or hate are not really that important, though at the moment we are caught up in them, we think they are. They are mainly scapegoats or excuses for more activity of the ego/self, to prevent ego death. Any object or situation will do. To keep from appearing foolish, superficial or unwise the ego comes up with all kinds of good sounding reasons, justifications and convincing arguments for why it needs to acquire something or react to something. This could be an underlying reason why yard-sales and garage-sales are so popular in America. People have attics, closets and garages full of stuff they don't use any longer, and not because it is necessarily worn out or broken. Some of it--clothing, toys, gadgets, tools etc. were probably not used much or perhaps never. But people need to empty out their closets, garages etc.- in order to make room for more!
Many activities like shopping trips, hobbies and other pastimes are clever activities to preserve the sense of Self. Even getting upset, irritated and angry at others, often for trivial things, is just more excited energy to make the ego seem more alive. The unfortunate irony is that at the same time many of these activities and emotions entail a lot of un-necessaring suffering for ourself and others. So we can see this direct connection between Ignorance, Craving (clinging to Self) and Suffering.
This is why it is so difficult for the average person to quieten their mind or to experience total rest. One of the hardest things for the average person to do is to sit still in the middle of the day, close the eyes, not move any part of the body, and not go to sleep. After a few minutes they would most likely become increasingly restless, wanting to do something. This is one of the reasons why many people have difficulty going into deeper states of meditation. The ego shrinks away from the increasing body and mental stillness because it feels like death-ego death.
'I' consciousness arises as a resistance to the flow of impermanence coming through the senses. Resistance manifests as attraction or aversion to sense stimuli, including our thoughts, memories, emotions etc. When attraction and aversion subside resistance also subsides, thus weakening the strength of I consciouness. This ratio or corresponding amount of resistance to ego consciousness can be directly observed during meditation. In following this train of observation one could say that the sense of 'I' is nothing more than resistance (grasping and aversion) to what is. One could even give a mathematical equation of suffering as follows: S=PxR or suffering = pain x resistance.
Desire and the ego is also directly related to the past and future. When we see, hear, smell, taste and feel touch on/in the body the conditioned mind automatically brings in our past memories of attraction/aversion with these same or similiar sensations and objects. Within miliseconds these are projected into the future causing similiar or related susequent thoughts, emotions and reactions to arise in a chain reaction. The mind constantly moves between the past and the future in this way creating the illusion of time. It is like the pendulum of a grandfather clock that keeps swinging from right to left. If the pendulum stops swinging, the time on the clock stops. Both the ego and time are simultaneously created and regenerate moment after moment through the deepest inner activities of the mind powered by ignorance and desire.
The practice of mindfulness and vipassana meditation is a mental training that keeps attention in the present moment, remaining aware of whatever the body and mind is doing moment by moment. One tries not to allow the mind to get carried away with attachment or aversion to sense stimuli or get lost in thoughts. The calm but alert attention is gradually tuned into the flow of impermanence continually arising and vanishing through the six senses. One developes the ability to simply watch and let go of resistance to discomfort or pain or other distractions while opening up and relaxing more and more into the ongoing present moment, without reacting or thinking of the past and future. At some point the feeling of time, desire, the ego and suffering will all dissolve. This is direct experience of the Dhamma, The Four Noble Truths.