Sonnet #30

Sonnet number thirty appears to be a reflection on past regrets, mistakes, or losses. It's really a depressing sonnet until the very end. The speaker says "then can I drown an eye (unused to flow) for precious friends hid in death's dateless nights." This means the speaker is lamenting (or crying, which they don't usually do) over dead friends, but at the end it says "but if the while I think on thee (dear friend) all losses are restored, and sorrows end." This is saying that when the speaker thinks of this one specific person all his past troubles disappear. So I think the speaker is saying in times of desperation when they're wallowing in past misery and things that were long lost or let go, they turn to this person to make things feel right again.