I know that this was Life,—the track Whereon with equal feet we fared; And then, as now, the day prepared The daily burden for the back.
But this it was that made me move As light as carrier-birds in air; I loved the weight I had to bear, Because it needed help of Love:
Nor could I weary, heart or limb, When mighty Love would cleave in twain The lading of a single pain, And part it, giving half to him.
When his friend was alive, Tennyson could bear any weight (and we're talking metaphorical weight here—he means the everyday weight of duty, responsibility, etc., that people have) as long as he could share it with his friend.
He actually loved to carry that kind of weight because it was a burden he could split with his friend, and he could pick up Arthur's load, too.