We are instructed to love God first, because that is the necessary first step to understanding God's 'system' - I mean, we must first acknowledge that God is loving towards us, well-intended, doing His best etc.
Or, to be Christian, we must first agree with the aims and purposes of God's 'plan' for us; we must agree that this plan is benign - for our benefit: we must each of us join with God in this plan.
The second commandment reinforces that that this is a system in which love is primary - and we must understand our situation in that light.
In sum - the two great commandments describe the 'metaphysics' of Life: the basic organization and principles of reality.
'All the laws and prophecies' - which also implies the many detailed and specific statements of scripture - should be interpreted in this light (i.e. they 'hang on' the two commandments), In other words, when their meaning, intention, provenance etc. are understood correctly, they cannot contradict these two commandments.
And if we think that any particular law, prophecy or line of scripture does contradict either of the two great commandments, then we have in some way misunderstood that law, prophecy or scripture.
The two great commandments are a great simplicity at the heart of Christianity: comprehensible by almost anyone.
Once grasped, they are of great value in discerning the specifics of what is true theology or doctrine, and what is false or mistaken . Even when errors of teaching or interpretation are backed-up by power, knowledge and/ or logic; when something violates the two great commandments, this tells us that we should reject it.
The two commandments are, indeed, not intended as rules; but intended to be absorbed into the heart - to allow us to move spontaneously and surely through the complexities of life; directed by an inner guidance system that will err in specifics and temporarily - but over time, and in general, will keep us moving in the right direction.
Note added: The two great commandments could perhaps usefully be summarized as The Love of God, and The Rule of Love.