There is a difference between cheerfulness and levity.

Cheerfulness is serene and peaceful. Levity is light and trifling.
Cheerfulness promotes evenness of temper and equanimity of
enjoyment. Levity drowns sorrow and pain for a short time,
only to have it return again with redoubled power.

I do not deny that there are certain kinds of sinful pleasures
which piety spoils; but then it first removes the taste and
desire for them--so their loss is nothing to be lamented.

The Christian hope, and the promises and consolations of
God's Word, furnish the only true ground of cheerfulness.
Who should be cheerful and happy, if not one who is delivered
from the terrors of hell and the fear of death--who is raised to
the dignity of a child of God--who has the hope of eternal life--
the prospect of dwelling forever in the presence of God, and in
the enjoyment of perfect felicity? But no one would associate
these things with that frivolity, levity and mirth, which are the
delight of the pleasure-loving world.

The gospel of Jesus Christ has a remedy for everything in
life that is calculated to make us gloomy and sad. It offers the
pardon of sin to the penitent and believing; the aid of grace to
those who struggle against an evil disposition; and help against
temptation. It promises to relieve the believer from fear, and
affords consolation in affliction.

There is no reason why a true Christian should not be cheerful.
There are, indeed, many things, which he sees, within and without,
that must give him pain. But there is that in his Christian hope, and
in the considerations brought to his mind from the Word of God,
which is able to bear him high above them all.