I think that most people who don't know much about bipolar equate mania with feeling really happy and depression with feeling really sad. This is a gross distortion of the reality of living with bipolar. In this post I focus on the relationship between mania and happiness.
First of all, when you are manic you have a number of symptoms, only one of which is feeling euphoric. Other symptoms may include risky behaviour, racing thoughts, flight of ideas, decreased need for sleep, increased energy, distractedness etc. These symptoms are not just about the mood you are in but about your mental processes, your energy levels and your behaviour. In other words, bipolar is not only a mood disorder. Therefore it is not just a case of feeling happy.
Second, the euphoria experienced during mania isn’t just the feeling of being happy. It is an extreme emotional state whereby your thinking becomes completely distorted. You feel giddy and on top of the world like you could do anything. This leads to poor judgement, poor decision making and generally strange inappropriate behaviour. The feelings of euphoria are also often completely inappropriate to the situation.
In fact you don’t even need to feel euphoric to be having a manic episode. You may not feel happy at all but instead feel distressed by your symptoms. Some people (myself included) may present with all the symptoms of mania but instead of feeling euphoric they feel dysphoric and present with an agitated and irritable mood. In this case describing someone who is manic as happy is completely inappropriate.