Your own thoughts are what will determine your emotions, and by extension, much of your appearance to the outside world. If you have a lot of negative thoughts, people will be able to see it in the way you conduct yourself.
Moral Letters 75.11-12 states . I have often before explained the difference between the diseases of the mind and its passions. And I shall remind you once more: the diseases are hardened and chronic vices, such as greed and ambition; they have enfolded the mind in too close a grip, and have begun to be permanent evils thereof. To give a brief definition: by "disease" we mean a persistent perversion of the judgment, so that things which are mildly desirable are thought to be highly desirable. Or, if you prefer, we may define it thus: to be too zealous in striving for things which are only mildly desirable or not desirable at all, or to value highly things which ought to be valued but slightly or valued not at all. "Passions" are objectionable impulses of the spirit, sudden and vehement; they have come so often, and so little attention has been paid to them, that they have caused a state of disease; just as a catarrh, when there has been but a single attack and the catarrh has not yet become habitual, produces a cough, but causes consumption when it has become regular and chronic. Therefore we may say that those who have made most progress are beyond the reach of the "diseases"; but they still feel the "passions" even when very near perfection