It's usually leads to a heartache that prepares us for a mature love later in life. Largely teenage love is a by product of hormones and romantic fantasy. As a teenager we don't have much perspective about life. In fact we have not fully grown into the person we will eventually become. We often have not developed (self love) and we determine our worth by what others think of us. A teenager may be madly in love with someone who is cute, popular, and makes them laugh.
An adult wants to be with someone who can handle adult responsibilities, hold down a job, pay their bills, has plans for the future, is able to cope with streessul situations without taking it out on those they "love"...etc
Teenagers who meet at one another's locker, hold hands, eat together in the cafeteria, kiss and make out while their parents provide shelter, food, clothing, and spending money are in many ways living in a "fantasy world" which gives them permission to concentrate on each other in ways they would never be able to do as grown ups with adult responsibilities. We see the same thing happen on "reality shows" such as (The Bachelor) There's no stress from work, office politics, fear of lay offs, taxes, car notes, household chores, grocery shopping, rising costs of gas, and other bills. You throw in having a few kids in the mix and the focus really changes. It’s the absence of these responsibilities that intensifies teenage love.
Having so many responsibilities as adults and also having had some bad experiences in relationships makes it difficult to experience that (pure all encompassing love) we knew as teenagers. With maturity comes rational thinking which changes how we view romance and love. A lot of us long for those days in high school or college where we didn't have to worry about anything other than getting good grades and pleasing the one we love. Only the super rich have that kind of luxury where they can travel the world spoiling one another to death with love and affection without having to deal with the outside world constantly crashing in on them. Very few of us give 100% of our heart away again after the first heartache.