Even when he's happy (as he is here), Seth's basic instinct is still to cover up the emotion. The great news is that he's feeling more and more comfortable as time passes. In the scene below, he was so excited about his birthday gift (learning about his two riding lessons) that his joy started out as an involuntary smile. Within a sparse second, however, he quickly covered it up with a fist and tried so hard not to smile in an effort to gain control over his face (even as he's smiling in that first photo, his fist is poised, ready to cover his face). In one picture, you can see him pulling down on his face near his eyes - this is his attempt to (literally) wipe the smile off his face. Often at this point he buries his head under blankets or behind our backs or under our arms.
But, in a testament to the long way he's come in the past few years, you can still see that, despite his efforts, the joy is still there!