Like a lot of people my first "love" took place in high school. Unfortunately this is often the case. We are too young and to immature to really know what love is and we aren't living in "reality". It's easy to be in love when you all you have to focus on is school and that person. You have no idea how they would handle day to day streesses of working, paying bills, dealing with living together....etc
Essentially teenage love is not on par with adult love. As a teenager you have yet to figure (who you are) let alone know what to look for in a mate for life! What is driving your feelings are hormones and naiveté at that age.
Truth be told most first loves out of Jr. high school or Sr, high school usually don't lead to marriage.