What takes all the sadness away in Shakespeare's Sonnet 30?

In Shakespeare's Sonnet 30, thinking about his dear friend takes all the speaker's sorrows away.
The speaker addresses this friend and lists quite a number of woes. These include thinking about things past that will not return, considering how he has not achieved things he has sought, wondering about wasting his time mourning over what he didn't achieve, and remembering friends who have died. The weight of the woes he recounts testify to the power of his friendship; it is remarkable that simply thinking about his friend can counteract all these many sorrows. The idea of having his friend perks him up and compensates for his losses.
While the sonnet seems weighted with sorrows, the speaker shows his positive attitude in focusing on the value of what he has in the present moment—his friend.