Laughter is a sign of a joyful heart.
It can also be a sign of a humble heart.
If we really understand how amazing grace is, how amazing it is that God loves us in Christ, how He fully and freely forgives us and owns us forever, we should be able to laugh.
If we realize that we have nothing in ourselves to defend, that the cross has already fully condemned us and then fully released us from our condemnation, we should be able to laugh. We should be able to laugh at ourselves and all the idoisyncrasies and quirks so common to us all! There is much about life that is funny.
A constantly somber Christian is a contradiction.
Check out some funny stuff and laugh! http://www.savethewheel.com/melville/