I think since our real self and real emotions are actually real, they have an easier time being recalled, also emotions are connected to to the hippocampus which is involved in making memories. I don't know about anyone else, but when I try to fake an emotion I'm almost on autopilot, my real mind is in another place and I'm simply mimicking the other 'happy' people around me, you know, smile when they smile, nod when they nod, laugh when they laugh. People say it's easier to be around me like that, but i'm not really there, I'm like a puppet.