1 John 1:1-4 (ESV):
That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life—  the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us—  that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ.  And we are writing these things so that our joy may be complete.
John's summary of what he has to say about Jesus drips with a sober awe and joy. He is careful to note the very concrete evidence upon which he founds the astonishing conclusion that "this is the word of life." He saw the evidence, he heard it, and he touched it--because it was all wrapped up in his dear friend, Jesus the Messiah. And the only conclusion John could reach was that this dear friend was "the eternal life which was with the Father and was made manifest to us."
John will not, however, rest content with his own fellowship with Jesus, or with his own joy in Jesus. The fellowship and joy must be shared. Invitations must be issued. And that is precisely what John does here: he says, "come enjoy fellowship with us. Look over our shoulders and behold the word of life made manifest in my friend. In seeing him, and in knowing him, you will have fellowship not only with him but with his Father as well."
And when we ask, "why, John, do you invite us to do this?" he says, "so that our joy may be complete." At first that sounds strangely self-centered--John and his fellows are writing to complete their own joy, and not that of their readers? But another moment's reflection makes us grateful that the one issuing the invitation is so excited about the fellowship into which he invites us, and that he is so eager, so happy, to have us. And that kind of joy, the kind which is consummated in sharing, is very contagious.