Cheerfulness is a very great help in fostering the virtue of charity. Cheerfulness itself is a virtue. Therefore, it is a habit that can and should be acquired.
Cheerfulness is perhaps best represented in the word affability. St. Thomas Aquinas places affability under the general heading of the cardinal virtue of justice, the virtue that prompts us to give to others what is their due under any sense of duty or obligation. You are obliged to help and not hinder others around you in the world on their way toward Heaven. Not only are you to help the needy by your alms, and the erring by your advice, but you are also to help all whom you know or meet by your kindliness, pleasantness, and affability of manner.
Cheerfulness of attitude and manner is a great help to those who come into contact with you. If you are a sour, unsociable, gloomy-looking person, you will make people feel uneasy, and you will intensify your own temptations to give way to sadness. On the other hand, if you are cheerful, you will lift the spirits of people, invite their confidence, and increase their hope of serving God well.
If you consistently present a gloomy attitude toward life and everybody around you, it may be because you are suffering from a case of self-pity. You let your sorrows and misfortunes overwhelm you. Or you may be prompted by envy to refuse even an effort at being cheerful because you are thinking of the many good things others have that you are denied. Or you may be a victim of your feelings. Temperamentally you may be inclined toward sadness, and you take the position that you should let your temperament rule you.