Well, we think about it, and we see that the usual ideas about what the virtues of a human being are a accurate. The human virtues are things like courage, justice, patience, etc. If you are reasoning well you will be these things. You will see, for instance, that you cannot function well as a human if you do not have courage, and that having courage involves correct reasoning and judgement, in particular, correct reasoning and judgement about myself and my capacities, the situations I am in and what they in fact demand, and so forth. The aquisition of such virtues requires training, so that they become something like habits, namely, habits to choose how to act in a practically wise or reasonable way.
Virtues ARE precisely such habits or dispositions.
If the trainers know what's what and can train me into the virtues, I have a good chance of being happy. And it requires that I percieve the situation accurately. Kindness requires having sympathetic feelings and being perceptive about people and their needs. And so forth. Happiness, it turns out, is not a set of feelings but having a set of dispositions to act, feel, and percieve in a certain way.