Jaques' empathy for the deer is a strength as a human, but a weakness for someone trying to survive in the forest.
Jaques' alternative perspectives allow him to generate good advice, a strength.
Jaques' pessimism is a weakness in someone trying to survive in the forest, but, again, it provides balance for the company, and so is a strength.
He can be clear-sighted, quite a strength.
As for motivation, he wants to serve the Duke, and then, later, to find a greater meaning in his life.