1. You don't feel the need to see each other every day...
...but it would be nice.
2. You start to sound alike.
That whole "picking up each others slang" thing is no joke.
3. You begin to be able to communicate without words.
They know what each and every one of your facial expressions means.
4. You are always invited to things as a couple.
It becomes rare to be invited to something as an individual anymore.
5. Their friends become your friends, and vice versa.
Since you're always considered an item, you begin to have a lot of mutual friends.
6. PDA is a no-go.
It's also pretty rare for the two of you to kiss and hold hands in public after being together for a long time. And you kind of get weirded out by couples that do show PDA.
7. Talking about the future becomes "we" or "us", instead of "me" or "I."
You pretty much plan on them sticking around.
8. You are on a whole new level of comfortable with each other.
...and it is the greatest thing you could imagine.