Dear friend, far off, my lost desire, So far, so near in woe and weal; O loved the most, when most I feel There is a lower and a higher;
Known and unknown; human, divine; Sweet human hand and lips and eye; Dear heavenly friend that canst not die, Mine, mine, for ever, ever mine;
Strange friend, past, present, and to be; Loved deeplier, darklier understood; Behold, I dream a dream of good, And mingle all the world with thee.
Directly addressing Arthur, Tennyson calls him "my lost desire." You guessed it. This continues the ambiguous way the poet has addressed his friend throughout the poem. Friends, or more? Is it maybe just that poets don't have a way to neatly explain such a deep friendship between two men, so they fall back on tried-and-true romantic love tropes?
At any rate, he develops the idea of Arthur being a melding of various things: known and unknown; human and divine; and someone existing in the past, present, and future. It's a sort of cosmic representation of everything Arthur represents—he's the whole world to Tennyson.
And Tennyson sees the entire world as Arthur. He "mingle[s] all the world" with him.
Through his grief for his friend, he's learning some larger truths (and we mean Truths with a capital T) about the world.