Because you have already practised it for a long time. It is the momentum of old habit. If you allow something to happen again and again, it becomes very powerful.
That’s why we need to change the pattern of the mind. We try to be aware all the time; then, the mind cannot think. If you’re not aware of the present moment, the mind is thinking or day dreaming all the time.
The mind thinks so much because it wants sensual pleasure. We have enjoyed our thinking for a long time. Because of this habit, the momentum is there. That is why we try not to follow or allow this – we need to be aware all the time to change this old habit.