James 2:9 But if you show favouritism, you sin and are convicted by the law as law-breakers.
Is favouritism a sin in itself? No. But when we do show it we often get caught up in our own feelings and put aside the law!
The sin is turning our backs on the law that God gives us just so that we can do what we want to for someone else; that is actually lifting up the person above God.
God has already told us many times that we should not lift up anything or anyone above Him because He is Sovereign God.
Love that special someone, but love God first of all.