Pessimism yields bad results. If you prune away all action, you have no action to pull on the bootstraps. Do something, and do it well, and this can at least possibly lead to good things. Do it with a conscious mind and that's more likely to lead to better things for that mind than bad things. Soon optimism is not so irrational: there's no reason to think any course of action will be good, but there is reason to think pessimism will go nowhere. Optimimism is better than dead even.