This sonnet is about a speaker (who we assume to be Sir Philip as readers) who is struggling with himself in a thinking mood. He reports that his quest for thought could come from a variety of places. One source may be his own "melancholy." Some think it could be from his own knowledge. Others believe his thoughts come from his service as a prince. Yet a final group of judges believe his thoughts come from his ambition. Although all these others weigh in on the source of the speaker's thoughts, the speaker finally admits that his thoughts can only stop or start with his "Stella's eyes and Stella's heart."
This would be a legitimate interpretation for the time period as Sir Philip Sidney was not noble, but found himself close to the court of Queen Elizabeth I. It also rings true for humanity as when people deal with the results of being in love, all parts of their minds are consumed. Even though they attend the duties of work, thoughts of love work their way into the stream of thoughts for all people in love.
In terms of poetic devices, this poem is in sonnet form with an abbaaccaddeeff rhyme scheme.