It can't be that she's so happy and is genuinely finding lots of things very funny. Watch any recent interview. She's clearly doing something she's come to believe she needs to do. Telling women to smile... it's a well-recognized feminist issue:
What if Hillary's natural demeanor were as angry looking and sounding as Bernie's and she just let it show? It would not work, would it? That's why she's decided she must smile and laugh and put in all that effort. Think about why it wouldn't work. If you think, it's because she's a woman, she'd be perceived as a shrew and hated, congratulations, you're a feminist. There are other answers, of course, and I expect to see them in the comments. Here. I'll get us started:
1. Natural demeanor? There's nothing natural about Hillary. Anything she might attempt to do now would be fake and properly rejected as fake.
2. Whatever anger she's got in there would not be the refreshing, righteous anger we get from Bernie. It would be horrifying and disgusting, and that's why she doesn't show it and why we'd recoil from it.