And if you did not wish to be a better person in the past, then you have changed already. From such intent comes thought, then speech and action. But in my opinion better isn't a goal you reach. Better is having that intent in your mind, and applying it to whatever you do. It is, simply, adding love. Patience instead of annoyance, aggravation and anger. Love instead of hatred. Charity instead of attachment and greed. In this, every moment is a test, a lesson, an experience. When faced with a decision, how do we act? When a thought or emotion we consider bad arises, how do we deal with it? Do we shift away from it, or allow it to control us? Some of this is not easy, and I certainly fail at it myself quite often, but I think the true manifestation of better is always in the present, now, with whatever circumstances we have, which are never optimal.
By saying "I am an optimist" or "I am a pessimist", you limit your thinking and behavior, and when you lose the state you identify with -- and you will, because they are not permanent -- you experience the suffering of change. In reality, you are capable of both, and you are neither, and neither is you.
Ok, enough preaching

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It's a possibility, but is pretty rare in the clinical trial data for antidepressant medication. If indeed the medication is triggering it, you could talk to your doctor and he could make changes to try to reduce or eliminate this side effect. I'm probably just being optimistic, but I'd like to think you could've made changes on your own here too resulting in greater happiness for yourself.
Is it intrinsically irritating, or irritating because you think you shouldn't feel happy, but should feel a different way instead? (ok ok, enough metaphysics of dualism for now

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