Shakespeare uses these lines as yet another way to portray Romeo as a poor romantic fool, who gets carried away by his emotions. Romeo is being ironic when he tells Juliet that he is no pilot. Juliet has just asked him how it is that he managed to find her on her balcony, at her home, inside the garden walls. Romeo's reply is to say that while he normally does not have a good sense of direction, love lead him to her, showing us just how carried away he can get by his emotions.
When Romeo first responds to Juliet's question, "By whose direction found'st thou out this place?," he says, "By love, that first did prompt me to inquire," meaning love gave him directions to Juliet, since he fell in love with her and asked love where to find her (II.ii.83-84). He next says, "He lent me counsel, and I lent him eyes," meaning that love counseled him into loving and finding Juliet, so Romeo gave love his eyes, so that love could find Juliet (85). Romeo next says, "I am no pilot" (86). Pilots, or sea captains, need a good sense of direction in order to steer a ship across a wide ocean. Hence, when Romeo says that he is "no pilot," or not a pilot, he is saying that normally, he has no sense of direction. Normally, he would not be able to find anything, but through love, he found her.
His argument that he is no pilot allows him to extend his metaphor further to say that, even though he is no pilot, he would have been able to navigate to the farthest land across the farthest sea to find her, due to her worth, as we see in his last lines:
I am no pilot; yet, wert though as far
As the vast shore wash'd with the farthest sea,
I would adventure for such merchandise. (86-88)
Hence, we see that in saying that he is "no pilot," Romeo is saying that he normally has no sense of direction and no navigation skills but was able to find her any way, due to love.