Greed is bad in any form. There are times you receive a particular favor from the Lord, such as a special feeling of joy or an inspired thought, and wish to have more of the same. This to me is spiritual greed. The danger is that you seek these favors rather than the Lord himself. If you receive them, be grateful but don't get elated. And if you don't receive anything special from the Lord, be equally content for you already have the Lord himself.