I believe God is the most supremely happy being in the Universe. He has never lacked in anything, and He has always been completely satisfied and fulfilled in His triune-relationship of Father, Son and Holy Spirit. God has never been in want or in need. He is ultimately satisfied in Himself and he is the only one who can do so.
He lacks nothing and He is always joyful.
Yet, is it possible for God to become even happier? I believe so; for just as he is grieved by sin and the damage we do to one another, and our rebellion against his good rule, he is overjoyed when we turn to him in faith, love, an repentance. There may be no clearer image of God's joy than in Luke 15, when Jesus describes the courts of heaven at the return of a sinner to relationship with God:
"Likewise, I say to you, there is joy in the presence of the angels of God over one sinner who repents.” (Luke 15:10 NKJV)
I've always read this passage and imagined that it was the angels rejoicing before God, but that is not what the passage says. It states there is "rejoicing in the PRESENCE of the angels". So then, who is the One in the presence of the angels? Who is the One rejoicing? It appears that the God who made you and me, though we rebel against Him by nature and by choice, celebrates and rejoices before the Host of Heaven whenever we repent and return to Him.
When you return to your Heavenly Father through faith in His Son, you fill the halls of Heaven with happiness.