You're heard the phrase "fake it til you make it" right? Psychology studies have been done that shows that if you smile, even a fake smile will start to lift your spirits. So maybe try faking it. Act really energetic. Make the muscles on your face move into the widest smile you can, and keep up that effort when you interact with others. I think being happy is about building stimulating memories along with body and muscle memory, so if you want to get out your rut, you've got to try to get out of it by forcing new behaviors.