The differences between Romeo and Juliet in Shakespeare's play by the same name make their relationship work in that each supports the other and the relationship is allowed to grow.
At the start of the relationship, Romeo is an immature youth who thinks he is in love with Rosaline. He moons around obsessing about a young woman who has no interest in him—nothing else.
In Act One, scene one, Romeo sighs and complains:
Oh, my. Time drags when you're sad...Griefs of mine own lie heavy in my breast...What else is [love]? A wise madness...A deadly poison...In sadness, cousin, I do love a woman.
When Romeo meets Juliet, we see that he finally knows what true love is. Where he is at a loss (or without a real desire) to know what to do about Rosaline, having Juliet is not a passing fancy, but a mature and serious dedication to the woman he loves more than life itself.
Juliet, on the other hand, is a strong young woman who is forced to stand up to her father who wants her to marry Paris—who she does not love. And ultimately she is married to Romeo anyway and cannot marry Paris even if she had changed her mind. Had Romeo approached her like a wounded dog (as he was acting over Rosaline), I doubt she would have given him a second glance. One thing that may make him so appealing is his unswerving dedication. He ends up being the only one she can count on as her father pushes her to marry Paris. Her mother, the Nurse and even Friar Lawrence end up letting her down in the end. Romeo, as the man he becomes, is the one constant in her universe.
The reader sees Juliet's spunk when she agrees to marry Romeo in secret. She does not consult her parents or talk to her friends, but pledges her love to Romeo when he seeks her out, at her house:
I gave thee [my love's faithful vow] before thou didst request it,
And yet I would it were to give again. (II.ii.127-128)
In Act IV, scene iii, Juliet is brave enough to take the Friar's potion to simulate her death even though she is afraid she might be buried alive or that it might kill her. She even has a knife nearby in case it doesn't work:
What if this mixture do not work at all?
Shall I be married then tomorrow morning?
No, no. This shall forbid it. [She lays down her knife.]
Though Romeo is weak to begin with, he grows up quickly once he falls in love with Juliet. He is willing, also, to take his life if they cannot be married. In the end, believing she is dead, He does. He is deeply devoted, though this was not the case with Rosaline.
Juliet does not spurn him, but takes a chance on Romeo. She is not fearful. She defies her parents and marries in secret.
...Juliet, a heretofore inexperienced child, suddenly speaks with great naturalness, insight, and understanding about love.
It would seem that their differences affect the other in a positive way and they quickly become totally devoted to each other. Their love only gets stronger as each of them decides what he/she wants out of life, even as they defy convention to do so.
Even though Romeo's speeches about love early in the play are...somewhat awkward, they show that...[w]hen he first sees Juliet, he shows that he is able to appreciate true beauty ...
While they are both young, Romeo and Juliet bring the best they have to offer to the marriage table. While Romeo may seem weak and ineffectual, Juliet's presence buoys him up. And Juliet shows the depth of her devotion to Romeo, allowing the relationship to flourish.