Why does the Holy Spirit come from the Father, and not from Christ who has already told His Holy Disciples that when He leaves, He will send them the Comforter. Besides, what is the reason for all this, when the Three are in One and Each does a certain job?
I need to clarify the use of some terminology in order for you to understand this issue clearly.
The verb "proceed" as used in the Holy Bible and the Creed is often confused with "to send." We should not confuse the eternal Procession of the Holy Spirit with the temporal mission of the Spirit. The Procession refers to the Holy Spirit's relationship to the Father - the Spirit proceeds eternally only from the Father. The Sending of the Holy Spirit into our time and space is an entirely different matter.
Our Lord Jesus Christ specified, "But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me" (John 15:26). The Holy Spirit does come from, and through, the Son, in the sense that the Spirit is sent into the world by the Son. However, the Father is the Unique Source of the Godhead, the Son eternally begotten by the Father, and the Holy Spirit eternally proceeding from the Father, are also equally God.
To claim that the Holy Spirit proceeds from the Father and the Son places the Holy Spirit in a state of subordination to the Father and the Son. The Holy Spirit is Consubstantial with the Father and the Son, because proceeding from the Father, the unique source of the Deity, the Holy Spirit has His own existence and personal function. His task is to bring about the unity of the human race in the Body of Christ. All our liturgical services include prayers to the Holy Spirit; with invocation of His name in order that the sacrament of the Eucharistic be effected.