Who is in control of the love story between Orlando and Rosalind?

In Act 1, Orlando and Rosalind meet at a wrestling match. After watching Orlando defeat a formidable opponent, Rosalind is impressed and congratulates the young man; she even gives him a chain to wear around his neck. Orlando is smitten with the beautiful Rosalind immediately, and Rosalind later confides to her cousin Celia that she feels she is falling in love with Orlando.
So it seems that their love story begins spontaneously—what you might call "love at first sight" or an act of fate or destiny.
In Act 3, though, it is Rosalind who takes control of the relationship. She and Celia have fled Duke Frederick in disguise and are hiding out in the Forest of Arden. Rosalind finds sappy love poems written on various trees and then learns that it is Orlando who is writing them, as he is also traveling in the forest. When Rosalind and Orlando's paths cross in the forest, she makes a decision to engage with him while in her disguise as the male character Ganymede:

I will speak to him like a saucy lackey, andunder that habit play the knave with him.
(III.ii.292–293)

Rosalind sees this deception as an opportunity to get to know Orlando in a more authentic way. She tells him that she wants to help cure him of his forlorn lovesickness, but her true intention is to test him—to see if the two of them are really compatible and to see if he truly cares about her, beyond loving her feminine charms and beauty. Celia doubts Orlando's affections are genuine and lasting, and Rosalind wants to prove her wrong.
Consider the two characters' demonstrated abilities as writers. Orlando is inarticulate in speech and unconvincing in his writing; he pens bad poems full of cliches. Rosalind, on the other hand, improvises a very convincing portrayal as the character Ganymede and succeeds in manipulating the actions of others around her, proving herself to be a very competent author indeed. A smarter and more sophisticated storyteller, Rosalind is the one to control her love story with Orlando, whom she decides is worthy to be her husband in the final act of the play.