It's cliché to think a Stoic is not joyful. This comes from a misunderstanding of what it means to be Stoic.
Stoicism aims to help an individual to be resilient; to have and retain equanimity. This does not mean they are always sad or always ebullient. But rather, the goal is to be steady in joyfulness or happiness or to always have a good spirit about you - eudaimonia.
Seneca exhorts Lucilius to focus on being of a sound mind at all times.
The "foundation" of a sound mind is to "not find joy in useless things." What are useless things? These are "externals" - those things that lay beyond your control. You control the "internals" but you cannot control the externals.
Seneca notes externals which should not disrupt your equanimity: death, poverty, pleasure and pain.
Real joy, believe me, is a stern matter. Can one, do you think, despise death with a care-free countenance, or with a "blithe and gay" expression, as our young dandies are accustomed to say? Or can one thus open his door to poverty, or hold the curb on his pleasures, or contemplate the endurance of pain? He who ponders these things in his heart is indeed full of joy; but it is not a cheerful joy. It is just this joy, however, of which I would have you become the owner; for it will never fail you when once you have found its source.
The contemplation of your death or your poverty; or your forbearance of pleasure and your endurance of pain is your work and practice for becoming Stoic - which is synonymous for always having a good sprit - a good flow - a steady joy about you all the time. It is not easy to do. This is why there are so many practices in Stoicism. These are designed to help you be resilient and joyful all the time, regardless of circumstance. They also teach you to rely less on externals and to help place your center for desire within yourself rather than something that could come or go in your life.
Externals are superficial and fickle. But if you center your joy on doing what is right, from a virtuous perspective, then you will have dug deep enough to find an unending source of joy.
The yield of poor mines is on the surface; those are really rich whose veins lurk deep, and they will make more bountiful returns to him who delves unceasingly.
Marcus Aurelius expressed a similar sentiment when he said,
Dig inside yourself. Inside there is a spring of goodness ready to gush at any moment, if you keep digging.
Seneca further advises,
cast aside and trample under foot all those things that glitter outwardly and are held out to you by another or as obtainable from another; look toward the true good, and rejoice only in that which comes from your own store. And what do I mean by "from your own store"? I mean from your very self, that which is the best part of you. The frail body, also, even though we can accomplish nothing without it, is to be regarded as necessary rather than as important; it involves us in vain pleasures, short-lived, and soon to be regretted.
This life of joyful equanimity can be yours if you put in the work. You have to practice, live your life, introspect and learn from mistakes in order to improve. Just reading quotes or motivational posters won't cut it. The inner heavy lifting must be done to show outward gains. And get on with it! Stop making plans to become better. Be better today.
They live ill who are always beginning to live.