Give two reasons why the fourth line of the second stanza of Suckling's "Song" ends with "do't" instead of "do it."I understand that using "do it"...

Two reasons that Sir John Suckling used do't instead of do it could be: because he wrote in a rustic dialect

And because he was writing cynically about love in this poem, the abbreviation adds to the disrespect that he feels for his lover.  It gives a more gritty feel a simpler expression that the more formal do it.  Abbreviations like this one appear in his other poems also.

 "This familiar song from Aglaura, illustrates again the poet’s skillful use of alliteration and assonance, as well as the effective change of tone and rhythm he can achieve in his conclusions."

For more information about Sir John Suckling and his poetry, click on the first link.