There are so many wrong things about the love that Romeo and Juliet had. They said they loved each other, but I do not find that possible. For one thing, the love that Romeo felt for Rosaline was not love. Second, Shakespeare does not exactly explain love very well, or the definition of it. Thirdly, Juliet and Romeo were never in love, plain and simple. And fourthly, defining infatuation. Apparently, Shakespeare has mistaken love for infatuation in this play.
First off, the 'love' that Romeo felt for Rosaline was definitely not actual love. He only strongly admired her from a distance, not being able to get closer. She never felt anything for him in return; it was never meant to be. But when Juliet came along, his feelings were strong enough to forget about Rosaline completely. Juliet was different; she actually had feelings for Romeo too. When Juliet had feelings for Romeo, Rosaline barely even knew he existed. When Juliet spent time with Romeo and cared for him even though she barely knew him, Rosaline could not have cared less. The fact that Romeo and Juliet's families are enemies possibly might make the two teenagers want to be together more, to sort of rebel against their families' ways. I mean, they are teenagers.
Now, we have to go over defining love, because clearly it is not properly introduced in Shakespeare's play. There is no specific way to know you're in love, because if you actually are you should just know, and feel it. There's no telling that a first love will never last. There is no rule that you can only be in love when you are a certain age. Anyone is capable of love. Teenage love is true too, and can be strong. Or at least, some is. You cannot say that a person is only in love when they feel certain things. It only matters that you do know the person somewhat well, and you strongly care about them. That they bring out a positive part of you or make you feel warm and fuzzy inside, and that you feel no need