Why does romeo feel that he hasn't seen or experienced love until the night he saw Juliet?

Romeo is completely smitten by Juliet the very first time he lays eyes on her. Romeo's young, naive, and inexperienced in love, and so he's experiencing emotions he's never had before. He once claimed to be in love with Juliet's cousin Rosaline, but that was just a youthful infatuation; this, however, is the real thing. Then, Romeo's eyes had lied to him, because they had never seen true beauty:

Did my heart love till now? Forswear it, sight!
For I ne'er saw true beauty till this night. (Act I Scene v).

But now, the scales have fallen from Romeo's lying eyes and, in the beautiful figure of Juliet, he suddenly beholds a vision of loveliness the likes of which he has never seen before. Now he knows the true meaning of beauty, beauty that's too good for this world, beauty that transcends the merely mortal to ascend to an entirely different plane of being:

Beauty too rich for use, for earth too dear. (Act I Scene v).