The speaker personifies love as a male child who is always crying and demanding things. If you give him what he wants, though, he’s just more likely to leave.
This personification is also an instance of extended metaphor, as love will be depicted as this child throughout the poem. By representing love in this way, the speaker alludes to Cupid, the Roman god of love and desire, who is traditionally pictured as a winged boy. Importantly, Cupid is also traditionally described as fickle and mischievous, a creator of havoc and confusion.
Beyond this allusion, the representation of love as a specifically male child is significant. While the poem's “he” might refer to love in general, it can also be read as representing a male romantic partner. From the start, then, by depicting love as a demanding, unreasonable male child, the speaker implies that male romantic partners can act in demanding, immature ways.
The speaker then says that if you “[p]lease him”—if you give love, or a male romantic partner, what he wants—he is likely to “fly,” or leave. In other words, the speaker is saying that lovers (and men in particular) are unfaithful, and could leave at any time. The end rhyme between “crying” and “flying” implies that these two actions are interconnected: for this speaker, love (especially male love) is demanding, but also fickle and untrustworthy.