Romeo says this in Act I, scene 1 after he comes upon the place where the servants of the Montagues and Capulets have been fighting. His remark highlights the close connection, seen throughout the play, between love and hate. While the servants have been brawling, representing hate, they are also doing so out of love and loyalty to the families they work for. Romeo goes on to use a series of oxymorons, or phrases that involve apparent contradictions. For example, he says, "O brawling love, O loving hate." These oxymorons emphasize the inherent connection between love and hate, a theme that will become more apparent as love develops between Romeo and Juliet, members of families who hate each other. Romeo can see the contradictions in the bloodshed between the two families and can see the way in which love and hate are so closely connected.
The line in Romeo and Juliet by William Shakespeare:
Here's much to do with hate, but more with love.
essentially describes the central theme of the play. In one sense, the play is about hatred between the Montague and Capulet families, a feud that has been passed down across generations. And yet, even more profoundly, the play is more about love, both the love between Romeo and Juliet and the way that love eventually triumphs over hate with the lovers' deaths finally ending the feud. The love of the Nurse for Juliet, the spiritual love of Friar Laurence for his flocks, the love of Romeo and his friends, are all major themes in the play. Thus the specific incident at the beginning of the play, serves as a microcosm of the play as a whole.
Romeo means that the events that unfold are greatly influenced by the hatred that the Montague and Capulet families have for one another, but are truly based more on the love that he and Juliet have. The effects of the two famlies' feud have affected everything that happens in the play, but the love of Romeo and Juliet can be held responsible for setting in motion (or at least encouraging) a chain of events that eventually lead to the reconciliation of the Capulet and Montague families. Everything that Romeo does is based on his love for Juliet, even though in doing so, he is forced to contend mindsets and actions that already existed; his actions are focused on love, but are undeniably affected by hatred.