Is there a facial or cosmetic treatment for a naturally sad looking face?

As you no doubt are already aware, without photos, it is virtually impossible to speak in anything but the most general terms. Some common causes of a sad or angry face include glabella frown lines and marionette lines around the mouth. But since you say that you do not have wrinkles, the possibility remains that early ptosis (sagging, falling) of the brow might possibly be the culprit in this case. If this is indeed so, "deep support volumetry," i.e. The Nonsurgical 3D Vectoring Browlift, may prove helpful. This entirely nonsurgical approach uses volumizing fillers to provide restorative support for soft tissue loss (i.e. to restore the youthful upward and outward flare of the outer third of the eyebrows) and to restore the outward projection and proper light reflection (of the sub-brow region). In my Upper East Side Manhattan office, I would use Juvederm Ultra Plus XC for this purpose. In my Israel satellite facility where a far greater number of regulatory agency approved fillers and volumizers are available, I would opt for Stylage XXL. The above technique requires advanced injector skills, so consultation with a board certified aesthetic core physician with extensive experience and expertise in the deep support volumetry technique is essential. Make sure to ask to see his/her befores and afters.

It is impossible to give a proper answer without seeing photographs. The most common reason in my 35 years of experience for a sad face is forehead sagging with lower eyebrow position requiring a forehead lift in order to correct this. See an experienced  facial plastic surgeon for correction. 

Without a picture or an in person examination, it is hard to give you any specifics.  Still, there are numerous possibilities that might help you.  I would suggest scheduling a consultation with a board-certified plastic surgeon who can examine you, discuss your goals, and review your options.