Many people think the reason spiritual teachers are always wanting them to control the ego is because ego makes us thing we are more important than we really are.  To some extent, that is true, but it is only one side of the coin.  The other side is ego makes us think we are less than we are.
    In the material or physical sense, a big ego can make us believe we are more important than others.  That our needs are more important than theirs.  That our opinion is more important than anyone elses.  Donald Trump is my favorite example of this type of overblown ego, but there are, sadly, many others.  
    The other side though, is that ego is very much a product of our physical brain and the mind it contains.  As such, it's knowledge is limited to the material world.  It has no knowledge of spiritual things, so cannot place any value on them.  As a result, the ego doesn't recognise us as being anything more than physical, material beings.  Just another animal struggling to survive.  
    That is why controlling the ego is so important for spiritual development.  We cannot know out true nature, our beginning, ore even our archtype if we let ego run the show.  This is what Jesus was trying to teach us in the little play with Peter when he tells Peter, who is playing the part of ego, Get thee behind me Satan (Matthew 16:23).  Jesus was telling Peter that he was saying the wrong thing because he was letting his ego, the tool of Satan, speak for him.