Whether or not we believe that Romeo and Juliet are truly in love by the end of Act 1, Scene 5, depends on whether or not we believe in the existence of love at first sight and how deeply we are willing to define that love. By the end of the scene, the reader is easily convinced that the two characters do indeed believe that they have fallen in love with each other, however, the reader may not be convinced that that love is real love.
We know by the end of the scene that both Romeo and Juliet are strongly attracted to each other and that both have interpreted that attraction as love. When Romeo first sees Juliet, he asks himself, "Did my heart love till now?" (54). His answer to himself is that he certainly did not know love until that moment because he "ne'er saw true beauty till this night," showing us that Romeo is equating love with an acknowledgement of physical beauty (55). In other words, for Romeo, love is only surface level. Romeo falls in love with beauty, and for him, that is true love. He thinks he is in love with Juliet, and we can agree that in seeing someone extremely attractive for the first time, hormones can make a person feel like they have fallen in love. However, since Romeo does not truly know Juliet yet, we can also see that this love he is feeling is not the most profound sort, nor is it the realest form of love.
Juliet's experience with love at first sight is slightly different from Romeo's. Juliet says nothing about Romeo's looks in this scene, so we know that looks are not the strongest governing factor. Looks do come into play, but we don't learn about that until the balcony scene, where we see her ask herself:
What's Montague? it is nor hand, nor foot,
Nor arm, nor face, nor any other part
Belonging to a man. (II.ii.)
All of these references to physical anatomy, culminating in an allusion to sexual anatomy, shows us that Juliet certainly is thinking of Romeo in a sexual manner, proving that she has found him as attractive as Romeo has found her.
However, when they first meet, it is his kiss and the touch of his hand, rather than his looks, that makes her feel drawn to him. When Romeo says that he will "profane" her hand by touching hers with his, she replies, "Good pilgrim, you do wrong your hand too much," showing us that she enjoyed being touched by his hand. When Romeo kisses her, she says, "You kiss by the book," meaning properly, or well, which is another clue that Juliet has enjoyed his touches. Hence, when Juliet tells her Nurse that she must marry Romeo, we know that her profound emotion is stemming from the hormones that were stirred by his touch and kisses. Juliet is young, and this is the first time she has been kissed by an attractive man. Therefore, it is only natural for her to feel like she is so in love with Romeo that she can declare to her Nurse,
If he be married,
My grave is like to be my wedding bed. (I.v.143-144)
Hence, by the end of Act 1, Scene 5, we are convinced that both Romeo and Juliet truly feel like they are in love with each other. We know that Romeo's feelings of love are based on physical attraction while Juliet's are based on physical touch. However, we also know that these feelings of "love at first sight" are not the strongest, truest feelings of love.