Answer: 1. He feels and behaves depressed and regretful about his past actions and thoughts. 2. The thought of his "dear friend" compensates his losses and his sorrow ends. 3. Because by changing his tone he makes emphasis the fact that his "dear friend" was indeed the light of his life.
Explanation: Shakespeare conveys a very sad and depressing message in the sonnet, regretting how he failed to achieve his goals, wasted the best years of his life, and crying over the loss of his friends. We can see this in the following lines: "I sigh the lack of many a thing I sought, then can I drown an eye, and with old woes new wail my dear time's waste.
Nevertheless, in the lines "But if the while I think on thee, dear friend, All losses are restor'd and sorrows end", he acknowledges his "dear friend", and the sonnet makes a twist. He emphasizes that thinking of this person relieves all his guilt and pain, making then, a tribute to them.
