Anyway on to some advice about Young Love in YA novels.
Sometimes an adult author writing for the YA genre can forget just how painful young love can be. Sure they've been through it before, but they may have forgotten the intensity. Remember young adults are feeling everything for the first time. Love is a new concept. The intense feelings and emotions that overwhelm a young adult can even scare them. The want to always be with that person and see them is new. Some handle it better than others while others can get obsessive. I've seen the stalker types at my old high school! Remember your first love and even your second. Note the differences. Young love for a freshman in high school is entirely different from the love a freshman in college feels. Make note of this.
Remember your first love in middle school or high school. How different that felt from the full blown thing in college? With each year and grade level love can intensify as young adults start to feel older. A freshman in high school thinks of marriage and college as some faraway concept. A senior in high school has college looming over them along w. many of its responsibilities. Obviously depending on your character and novel sex can come into play as the years progress in high school. Many different things can create deeper bonds between YA's so go hang out by the local high school and watch the couples.
You'll see the skanky/horny ones that make out until the last possible second. The shy ones who only hold hands, the older, mature couples who remind you of when you were younger. etc. Note the differences and remember there are different kinds of love and couples that all occur in different stages in high school. Experience and age play a big role. One 17 year old can be completely in love with their boyfriend and sleeping with him, while her best friend could still not have even had a boyfriend or her first kiss.
Know your character and assess what they have already experienced. Young adult love is different for everyone.