"Joy is the result of true acceptance of the gospel; but not the first result. Without consciousness of sin and apprehension of judgment there is no conversion."
Alexander MacLaren, Expositions of Holy Scripture, Vol. 8, page 145.
"And these are the ones sown on rocky ground: the ones who, when they hear the word, immediately receive it with joy. And they have no root in themselves, but endure for a while; then, when tribulation or persecution arises on account of the word, immediately they fall away." Mark 4:16-17
Joy apart from a knowledge of transgressions and sins against God and the corresponding guilty verdict before his law belies true salvation.True, gospel joy comes indeed when we are struck to the heart with guilt and conviction for our sins against God and then, by a miracle of the Lord, our eyes are opened to see the mercies of Christ Jesus. Such a joy will endure for it is founded on Christ himself.
Joy apart from a knowledge of sin is indeed nebulous and ultimately fruitless. Find true joy in the Gospel.