Their works to improve creation flow from their will to please God. Human beings have the capacity to sin largely as a result of having the capacity to plan. When someone plans their efforts, they need to be able to suppress their natural desires for a time. This is quite different from animals who live from moment to moment obeying their perceptions of the present and their instinctive drives. This is indeed a dramatic difference.
Human beings are able to look to the future - conceptualise it and form an intention to act.
This conscious intention can override even the most powerful of our instincts. Through it we have capacity to cause ourselves harm in the short term in order to realise the greater good in the long term. As an inevitable part of this we gain the potential to cause ourselves harm, i.e. the potential to sin. We cannot see clearly into the future. What we do instead is to believe in some future circumstances and direct our actions accordingly.