"It is the essential of love not to love self, but to love others, and to be conjoined with others by love. " and "Love consists in this, that its own should be another's; to feel the joy of another as joy in oneself, that is loving.But to feel one's own joy in another and not the other's joy in oneself is not loving; for this is loving self, while the former is loving the neighbour. These two kinds of love are diametrically opposed ..", as Swedenborg reminds us in DLW 47.
It would appear that if the Lord is all there is, then when He loves us, He is loving part of Himself. Don't you see that this is the love which He cannot have? There must be something in creation which is distinct from the Lord, although created and sustained by Him.
We read of the many (wonderful!) experiences of unity of the One Life that all mystics (and many others!) have had over the millenia. That is the way the world is!
I suspect that the desire to say that the Lord is 'all there is', comes from wanting to interpret the experiences of the One Life as 'non-dualism', following the advaita approach. You also want unity with that approach too.
However, a common meaning of 'dual' (which should be opposed) is one of a very fragmented world. We all know that this cannot be true! (i.e. should know!). Therefore often we need to replace everywhere 'non-dual' by 'non-fragmented', and 'dual' by 'fragmented'. This conveys the common meaning much more precisely, as often the explanation of 'duality' is fragmentation.
Then, we are able to find that in creation which is distinct from the Lord, which (whom) he can love without being selfish, and in whose joy he can delight. All of the wonderful mystical words describe these delights, and it would be a great shame for any presentation, if 'non-dual' were taken as the 'non-Distinct' nature of the Life from creation! The One Life is the life of creation, but is not equal to creation.
This is the reason for their being a creation, that all the delights of spiritual live and love become possible! This requires an idea of 'non-fragmentation' (sometimes confusingly called 'non-dualism'), along with an equally-important idea of 'Distinctness'.
One modern advocate of nondualism is Andrew Cohen. His nondualist philosophy can be examined from a theistic perspective, to see what is non-paradoxical and suitable for exoteric everyday application. He is found to have made significant developments in comparison with traditional Advaita Vedanta, and these developments clearly point toward a theistic viewpoint. For more details of this, see my article What is Enlightenment without Paradox? Nondualism from a theistic perspective.