As Miki says, "With happiness comes anxiety." Is their love really everlasting? So many people seem to be standing in their way of happiness (even more so in the anime version). And what if their love someday disappears, and Miki and Yuu each develop feelings for somebody else? Afterall, Miki really liked Ginta a few years ago, and Ginta really liked her back. He declared that his love for her would never change, but in the end, Miki ends up with Yuu while Ginta goes with Arimi.
Yuu also seems a bit idealized. He's tall, handsome, sweet, kind, and devoted to the girl he loves. So many girls try to tempt him, but he always stays true to Miki. (Even after they break up) As Anju once mentioned, "Yuu's a bit stubborn about the things he love." It'd be nice if guys like that existed in real life. The magic of anime and manga..creating near-perfect guys. Maybe my view is hampered by the fact that I live in a society where people often date and flirt around. One week a person can be in "love" with someone, and then that person falls in "love" with another person the next week. People might be more certain about their feelings in Japan, and stay in love with someone forever. It is just hard to visualize such a world where I live. Especially among high school couples.