“By Joy, therefore, I shall understand in what follows that passion that leads the Mind to a greater perfection. And by Sadness, that passion by which the Mind passes to a lesser perfection” (Spinoza, Ethics, book III, proposition 11, scholium, Collected Works vol. 1, Curley trans., p. 500-501).
This was always one of my favorite parts. Spinoza was the first writer I know of to explicitly give a positive ethical value in its own right to a joyful attitude (or any kind of passion). But this actually makes a lot of sense. When we are joyful we are more likely to be kind and generous, confident, and strong.
The sadness that he speaks of as leading to a lesser perfection has to do not so much with the bittersweet of mourning a loved one, as with unnecessarily negative attitudes in life. Emotionally negative attitudes lead to pettiness and spite, and make us into lesser beings. Intellectual criticism and questioning, of course, need not involve any emotional negativity.