Recently I have got this sensitivity to choose a singular noun v. plural one. For example, which of the followings are correct:
- Let's make friend with each other.
- Let's make friends with each other.
And
- We are friend.
- We are friends.
You say "Let's make friends," not "Let's make friend." The phrase "make friends" is an idiom, so it may not be easy to pin down the logic behind it.
You also use the plural in "we are friends." But this is the ordinary use of the plural, as in "John and Mary are friends." John and Mary are more than one person, so the plural is used. "We" likewise refers to more than one person.