When we try to think about something, the more general the thought, the less we are able to focus, to the point that our thoughts become meaningless.
Thought must be anchored in something tangible.
Hence, when a person is told to think about something, even about something holy, it is always simpler to focus on and deal with something specific.
The problem does not stem from a lack of concentration; it is simply part of human nature.
Since it is so difficult to turn our thoughts heavenward for more than a moment, we need some kind of focus, a point that we can grasp.
However, once this focus is achieved, it is very easy for it to deteriorate.
Instead of using this tangible point simply as a means of looking heavenward, one is liable to begin ascribing religious significance to the thing itself.
We tend, increasingly, to forget the goal and remember only the means.
Wherever we employ a means to an end in religious life, we must be extremely cautious, or the means itself may become an object of worship.
Maimonides maintains that this is precisely how idolatry first developed.
In his view, the starting point is always belief in God’s unity, but at a certain stage we begin to relate to the intermediaries more than to God Himself, until finally the center point is completely forgotten and we focus exclusively on the intermediaries (Laws of Idolatry 1:1-2).
Rabbi Adin Steinsaltz