1. You're always in lockstep with one another.
2. Even if you're the only one who wants to go out, you always have a (reluctant) friend there.
3. When you do something dumb, your roommate is always there.
4. Your friendship has survived disagreements and prank wars.
5. Most people think you're a couple on Facebook.
6. You've invented something together.
7. You've discussed going into business together on more than one occasion.
8. Your extracurricular group activities are always a labor of love.
9. You've started a prank war and have never once regretted it.
Well... maybe once.