We're learning creatures. But the knowledge we gain can't be predicted in advance. So if I'm aiming for a mental state I'm not going to know in advance all the features of that state. If I could, it would have to be an undesirable state since I wouldn't be learning anything.

I can only go for a feeling. Yet the choices of subsequent future mental states based on the feelings I predict they will elicit will depend on details of what I learned last time.

So we're not really aiming at mental states. Nor are we accumulating them because in reality we forget most of the details of our experiences.