The quote, "Whoever loved that loved not at first sight?," depicts love at first sight as an important theme in the play (III.v.82). For the most part, Shakespeare uses the theme to debunk the commonly held romantic notion of love at first sight that is especially prevalent in the pastoral literary genre, a genre Shakespeare satirizes in As You Like It. Some of the characters do fall in love at first sight in the play, but that feeling is actually not what continues to develop the love stories in the play. For example, Rosalind and Orlando certainly do fall in love at first sight; however, when Celia mentions the importance of character, Rosalind decides to test Orlando's love for her by remaining in her disguise as Ganymede because, disguised as a young man, Orlando will be much more willing to confess things that he would hesitate to openly tell a woman. Essentially, Rosalind uses Acts 3 and 4 as love "therapy sessions" to test their compatibility as a couple ("Love in As You Like It"). Beyond Rosalind and Orlando, Celia does not fall in love with Oliver at first sight but rather lets his new character be her guide. However, Oliver apparently does fall in love with Celia as Aliena at first sight. Hence we see that since we have couples both falling in love at first sight and resisting, Shakespeare is using the play to debunk the romantic ideal to promote being rational in love instead.
In the second act after Rosalind first falls in love with Orlando and Celia begins to tease her about it, one astute thing Celia says warns Rosalind not to fall in love so suddenly. After stating that Rosalind will eventually "wrestle" herself out of her emotions, she then says, "Let us talk in good earnest" and asks Rosalind if she really thinks it's possible for her to have fallen in love with Orlando so suddenly (I.iii.24-28). She further warns that just because Rosalind's father, Duke Senior, loved Orlando's father, Sir Rowland, so dearly, does not mean that the son is equal to the father and also worthy of being loved (31-32). Rosalind must have taken these words to heart, for next we see her challenging Orlando in Acts 3 and 4 to see just how much he truly loves her and how worthy he really is.
In contrast, Celia does not fall in love with Oliver at first sight because she has already heard much about Oliver's poor character from Orlando, as we see in her lines:
O, I have heard him [Orlando] speak of that same brother;
And he did render him the most unnatural
That lived amongst men. (IV.iii.121-23)
In this passage, the phrase "most natural" can be interpreted to mean the "most devoid of natural feeling," such as sympathy, love, and generosity (Shakespeare Navigators). Hence, Celia is stating that she has heard Oliver described as being particularly evil, and she has formed her initial judgement of him through this account. However, Oliver soon proves to her that he is a changed man, leading her to fall in love with him. Therefore, although Oliver may have fallen in love with Celia at first sight, Celia puts up some resistance, emphasizing Shakespeare's point to debunk the romantic convention.