1. I. Introduction/Apostolic Credentials (1:1-1:4)
Thematic Relation: John finds joy in bringing others to loving relationship with God.

Some Key Words

Manifested (phaneroo [5319]):
act of divine revelation, to make visible
Have fellowship (koinonia [2842]):
to have communion (common possession) or intimacy or be a society (persons united for a common interest.)
Joy (chara [5479]):
cause for rejoicing.
Made complete (pleroo [4137]):
to fill, as a net with fish. Fulfill or complete or perfect
 

Paraphrase

1 Jesus has always existed, but was for a time physically present, 2 to which the author was a witness. 3 He is sharing what he witnessed to bring his hearers into fellowship with himself and with God, 4 which will bring him joy.

Key Verse

1:3 - This verse reveals the personal experience of the author and the eternality of God, as well as indicating the purpose behind all that is to come in the letter: Fellowship with God.

Thematic Relevance:

 The love John writes of is rooted in the fellowship of believers, and of God.

Doctrinal Relevance:

 1 Jesus is eternal, 2 was physically revealed as having been with God, 3 whose Son He is.

Moral Relevance:

 We ought to proclaim what has been revealed to us.

Questions Raised:

 How is John's joy made complete?

Some Parallel Verses

1:1
Jn 1:1 - Jesus eternally God with God. Ac 4:20 - what they [John & Peter] have heard, they cannot help but speak. 2Pe 1:16 - their testimony is based only on what they have seen. Lk 24:39 - Jesus proved His physical presence by insisting on being handled. Jn 20:27 - which helped belief.
1:2
Jn 1:4 - life is in Christ. Jn 15:27- the Holy Spirit is a witness to the truth of Jesus, Jn 10:28 - who gives eternal life, Jn 17:3 - in revealing (manifesting) Himself, and the Father to man.
1:3
Jn 17:3 - knowing Christ Jesus and God the Father is to have fellowship with them (to know is to join in common cause with them). Jn 17:21 - which fellowship unites us as brothers and unites us with God. 1Co 1:9 - and which fellowship we were called to.
1:4
Jn 3:29 - the friend of the groom rejoices in the groom's marriage. This brings completeness to the friend's joy (answering our question).
 

New Thoughts

Our joy should be in bringing together Jesus the groom with His bride the church. This can only be accomplished by (a) revealing to those who don't yet see what has been made known to us, and (b) helping their growth. This is the purpose of God in our lives, and thus brings us into fellowship with Him. As we help others to find what we have found, they too join in that common purpose.
There's so much more here that I'm missing this time through. This just needs to be dwelt on at length.