  IIIp9 suggests that Spinoza is a psychological egoist of some sort. That is, it suggests that he believes that what human beings desire to do is to secure their own interests (construed here as perseverance in being). Indeed Spinoza goes on to define desire at IIIp9 as human striving (or appetite) together with the consciousness of striving. So clearly human desire for Spinoza is part of the striving for perseverance in being and thus shares its character.