To start out with, Kierkegaard marvels at the seeming fact that in all of ancient history the poets and philosophers never command that "you shall love". When thinking for himself, he discovered that he would never have come up with such a 'philosophy' on living life either, despite it seeming to be the true way to love.
The poetic version of love is a spontaneous love that is something less than Christian love because it makes an oath to love itself rather than something higher than love. Poets (and their readers) have a distaste for duty as it does not serve their emotional purposes, but Kierkegaard sees Christian love (and all true love, or love that is worthwhile) as a duty. Because of this duty, this "shall", love is eternal and unchanging - anything less than this is not Christian love - and will not remain as love. Kierkegaard feels that this undutiful love changes and morphs and thus is a cause of anxiety. Because spontaneous love changes it can morph into a number of other things: hate, jealousy, habit and despair. Dutiful love is the cure to anxiety (and the things spon. love morphs into) because it does not and can not change, it is secure and always love.
This means that Christian love does not require testing. Testing love, in fact, is not a sign of loves strength but a sure sign of it's weakness since the test is testing contingencies and is due to the already present anxiety about the changing nature of (spontaneous) love. Since no one can reasonably test for all contingencies, love (taken this way) is bound to fail. If love is a duty there are no contingencies to test for, one just loves, and therefore it never fails.
For example, when one (poetic lover) says to another, "I do not love you", the other (Christian lover) will not respond, "Then I will not love you either", but since love is a duty, in all humility he will say, "I will still love you, no matter if you love me or not".
The shall is eternal - there are no excuses.
You shall love your neighbour.