They sleep in your bed

Reciprocity

They let you see their butt hair. Yes, women too

You’re not desperate to hang out with them

Ask them.

When your saliva mixes and forms a perfect pyramid

Farting in front of each other

When he kissed you

When you can jack off to them fucking other people

You share personal or intimate details about your life or yourself that you don’t tell other people. Also, if you lick one another’s genitals.