If a theme is a message or idea that generalizes to the human condition and makes a statement about what it means to be human, there is no arguing the strength and universality of Shakespeare's tale of tragic young love thwarted by a family feud, because his is one of several versions of this story that dates back to the ancient Romans and Greeks. Among other things, Shakespeare explores the notion of youthful rebellion against well-meaning "we know what's best for you" parents, as well as the conflict between one's destiny being determined by fate, or individual choice (free will). Early in the play, we see Juliet's father embracing the "father knows best" role as he negotiates her marriage to Paris, saying she is as of yet too young, but that he will gladly turn her over to Paris in "two more summers". He also mentions that "The earth hath swallowed all my hopes but she", indicating that he has a great deal resting on the outcome of this marriage; one can assume he is referring to a grandson to carry on the family name, although that is not mentioned in this passage.
The youthful idealism of Romeo and Juliet on the topic of love reflects a popular Elizabethan concept called "courtly love". Practitioners of courtly love became immediately besotted with whoever was the latest target of their heart's desires; in Romeo's case, it was Rosaline, who he pined for, whined about, and generally made hiimself miserable dreaming of, moaning and groaning to anyone who would listen. So serious was this obsession that he dropped it immediately, or perhaps more accurately, redirected it immediately upon seeing Juliet.
Fate versus free will is introduced when Shakespeare alludes to the phrase "star-crossed lovers"; this idea that the choices the teens make may be irrelevant is threaded throughout the play, and makes its appearance through the most dramatic of irony when the play is approaching its climax; thinking Juliet is dead, when in fact, she has staged her death to escape to Romeo, Romeo ends his own life within seconds of Juliet's awakening from her "coma". However, even as he depicts this horrific irony unfolding, Shakespeare stays true to the idea that the kids have chosen their fates, in the sense that they have chosen to break themselves against the society in which they lived, ignoring every rule and social convention expected of them--particularly in Juliet's case--to be together, in life, and if necessary, in death.