In an argument with a friend, erratic emotional behaviour has been given as an example of unpredictability.
Actually, it depends on what kind of decisions we are talking about. For mundane everyday decisions: where to eat, do I go for a walk, etc, then maybe. If we are talking about important decisions rooted in our core values and general stance towards the world, then – definitely not.
As long as we can separate our psyche into emotional and rational parts or stereotype someone of being an emotional type or a rational type, I can say for certain that our important decisions are made by our emotions much more than our rational self.
An oversimplified picture of our emotional growth is that we are conditioned (or indoctrinated) as children with our emotional reactions (Jean Piaget), we are told and we imitate people what we are supposed to feel in a number of situations. Later having certain feelings (affects) lead us to attach emotions to them. After that during the teen years, we challenge the assumptions of these emotions and make a system out of them which much later we would call personality. Plus we add sexuality to the mix.
Rational growth is much more complicated: it is influenced by emotions, but it is based on our rational understanding of our environment – people, immediate environment, the world. It evolves for a much longer period of time and includes much more influences. people, books, experiences, etc.
I’m far from advocating the “lizard” brain as the base for our emotions (it is simply not a good model of emotions), but still, paradoxically, our emotions are much easier to predict than our rational stance, as long as important decisions are made.