It seems to me a HUGE part of social interaction is simply being able to superficially laugh along with the group. I - like virtually everyone - want to be loved and be part of the group, so I try to laugh along. When I'm not genuinely amused though (which is quite often), I have a smile that's downright painful. What do I do about it? Just having a neutral expression on my face, will result in me being regarded as humorless. That has happened in the past and being alone in a crowd.

And yet he was a fairly popular law student, because he learnt to smile and nod along with everyone around him.
He's proof that you're quite correct in a sense. The downside is that if you're always faking you never get any meaningful connection out of it. It would be a lonely life to always be a fake.
Learn to smile in a realistic way, which means with your whole face, including your eyes. For the most part though, just smile and laugh when it feels most natural to you.
Just find some people who are interesting. :)
I don't mind if he doesn't laugh at my jokes.
To be honest, I was offended when I was first getting to know him. I thought he didn't like me at all. Now I understand that his personality is just reserved so I don't mind. But he does smile every often :) Smiling is nice.
Or you could just do what shuggy said ;P
Because not everyone has to be in on a joke, its a natural permutation of social interactions that some people won't get it. Some people will not find a given joke funny even though everyone does, but that doesn't necessarily mean that social relations are bullshit. People are different. Thats what gives jokes value. Sometimes they'll land and everyone will share a laugh, sometimes nobody will laugh, sometimes only one or two people wont laugh. If everyone was always on the same page then humans would not be individuals, we would all be some sort of weird eu-social organism, like an ant hive or something.
And how would you feel if you were on the other side, the one telling jokes and you found out that the whole time people were laughing they were faking it? The only thing worse than not being funny to begin with is being under the false illusion that you are funny.
If you find something amusing you will find yourself laughing naturally at it. I'd keep it at that.