In the play Romeo and Juliet by William Shakespeare we see Romeo's mode of loving change from infatuation to a much more profound love. First he is in love with a girl called Rosaline and is miserably love-sick. Then he meets Juliet and his emotional life is transformed by excitement and affection. The thing that has changed is that with the second girl his love is reciprocated and this sems to release him from a paralysing spell of depression. he is released to fly high with Juliet - but not for long as their stars are crossed. Juliet's love for Romeo empowers him, gives him courage and determination to press ahead with his suit. Sadly, we never find out how well they will be matched, or how long the love will last.
Of course, there is no exact answer to this -- Shakespeare doesn't say "here's why." But my explanation is that he changes because of Juliet's love or because of his love for Juliet.
If you think about it, this whole play happens in just a few days. Romeo does not really have time to mature just as a result of growing older. So what has changed? The only real change is that he has met Juliet. That implies that loving her is what has changed him.
I think you could argue that this is part of what makes the play tragic -- he is improving as a person but does not have the chance to continue that process.
It's important to remember that Romeo is just 17, an adolescent in terms of his development, but an adult for his day and society. Romeo falls in love with Juliet, and backs it up with marriage, a very adult thing to do. As a married pair, they pass one night together, and consummate their union. So this is not a summer thing. Romeo evolves because of his marriage vow, and the personal risk he must pass through; before, during and after his marriage to Juliet, in order to be with her. Living his commitment, his attitude changes accordingly.