Sincerity 

is being honest

completely honest with another person

meaning what you say

or do

or show

Sincerity is so important

in relationships

dealing with any and every person

and just

but what becomes clear is that sincerity is loving  another person

and giving them what they are owed

Being sincere is giving

just as loving is giving

And although love is "all we need"

Without sincerity, love could be an empty promise.