How does the thought of his dear friend change the speaker's attitude?

Throughout the majority of the sonnet, the speaker sighs over "the lack of many a thing [he] sought" and cries about the time he feels he has wasted. He says that he does not typically cry, but he will weep over "precious friends" who have been lost to death, over loves that ended long ago, and for sights that he feels he will never see again. He grieves for things which he had previously let go, feeling the losses of such lost friends and loves and sights all over again, as though he had not already grieved in the past for these. The speaker mourns them all over again, as though each were a new, fresh loss. He seems to be caught in an unending cycle of sadness and loss and grief; the first twelve lines of the sonnet have a terribly melancholy mood. However, in the final two lines of the poem, the speaker says,

But if the while I think on thee, dear friend,All losses are restored, and sorrows end. (lines 13–14)

In other words, when the speaker thinks of one particular friend, he feels that everything he has lost is restored to him, and he feels no more sorrow. This is quite a change: the speaker will not grieve his losses forever; rather, he can think of his "dear friend" and feel whole and happy. The last two lines have a much more hopeful, optimistic mood than the previous twelve.