Why do we use plural from of verb when the subject is I or You?
Like when we say I love you, I is singular, isn’t?
Why do you think so? I love you, where I is singular and the verb accordingly is singular not plural 
Hi mishy,
Remember that in the present simple the only inflected form is the 3rd person singular: He/she/it loves. All the other forms, singular and plural are the same:
I/you/we/they love.
Alan