It has been said that agapao refers to "the love of God" and phileo is only "the love of [human beings]." But this distinction is only a very small part of the difference, and as such is in itself incorrect. Both of these words may convey intense emotion or may be relatively weak in their meanings. These words do not indicate degree of love, but kinds of love. Agapao refers to love which arises from a keen sense of the value and worth in the object of our love, and phileo describes the emotional attachment which results from intimate and prolonged association. That is why in the Scriptures we are never commanded to "love" with the word phileo. Even when husbands and wives are instructed to love one another, the word agapao is used, for it is impossible to command that kind of love which can arise only from intimate association. On the other hand, the saints are admonished to appreciate profoundly the worth and value in others, and agapao is used to convey this meaning. All Christians are not necessarily to have sentimental attachments for one another (phileo). This would be impossible, for our circle of intimate friends is limited by the nature of our lives. But we can all be commanded to appreciate intensely the worth of others.
-- Eugene A. Nida in God's Word in Man's Language