Practice, mostly.
You don't realise it at the time because when you're 16 and in love, you're flat-out convinced that the relationship is going to last 5eva after spending barely a week in each others company.
But when you look back on it a few years later, you realise just how important those early relationships were as far as:
- How to treat them.
- They're not all the same.
- Sort yo tongue out; you're trying to passionately kiss someone, not get a splinter out of your tongue using a brick.
- Forming opinions of the opposite sex (which, as per number 2, is often wrong)
- As per number 2 and number 4, making generalisations of the opposite sex based upon your limited experience is a big no-no...
- ...as is voicing those opinions to your newly found attraction (see number 2 for why you'll get laughed out of her bed)
- Etc
are concerned.
Relationships are a learning curve, and starting whilst you're in high school is a pretty good way to begin because it means you hopefully would've had enough practice (and learnt from your failings) at trying to get things right (whilst always bearing in mind number 2) by the time you're in your late teens/early 20s that you'll be able to have a serious and mature relationship.
You also learn how to find the clit using only your fingers, which is pretty much essential knowledge to anyone expecting to participate in a fumble in the dark.