"What Thing is Love" by George Peele, is about love's impact. Peele points out that, whilst love is "a pretty, pretty thing" it comes with its fair share of pain - "It is a prick, it is a sting." Love takes on real depth and character.
Peele is effectively warning men of the subtleties of love as it "creeps in at every hole." Men should be careful of "ladies' eyes." Once a man has been snared by "love's piercing darts," he will have his heart broken because love is so strong, even comparing it to a "mighty lord."
Men must be careful because they will think "With Venus he in heaven doth lie" when they place love (or a woman) on this pedestal or "mount so high." But "mars and she" (Venus), representing the woman, will play games of "even and odd" with men and confuse them, taking advantage of their lovesickness.
So the main theme would have to be the perils of being in love.