You Aren't Funny.

Not everyone can be funny, just like not everyone can be tall. Some people have a speech impediment. Others have a particularly slow mind that isn't suited for the intellectual rigors of wit. This is not your fault, you just need to accept it and try not to be funny. You can still contribute to the conversation, I'm sure you're a lovely person, but your attempts at humour make me stop laughing. This is the opposite of what everyone wanted to happen.
The unfunny person will generally fall into one of these categories:
The "Way Too Obscure". This person is very knowledgeable about a subject (or group of subjects) and on their own may be very interesting. I have a few friends like this, and while they are a great time when we're both ranting about politics or anime (I don't actually know that much about anime) or the pre-amps of American made guitar amplifiers from the 60's (this is a thing that I talk about), when you get them into general company, they are mood poison. The constant attempts to tie it back to their area of knowledge alienate basically everyone else, but they stare at me with the little puppy dog eyes and so I have to give a courtesy chuckle even though I'm thinking "jesus, dude, pick your audience."
The "Way Too Intense". This person gives a shit about EVERYTHING. They have endless reserves of enthusiasm and interest for any little thing that comes up, and try very hard to get everyone else feeling just as fired up as they are. This is the kind of person that screams "FUCK YEAH!" in a quiet pub when you mention you're grabbing crisps. No matter how much you try to indicate that this is neither the time nor the place for Mexican waves, the "Way Too Intense" will try to turn everything into a cause for celebration, from the fact that you agree on some innocuous piece of trivia to the arrival of a drink at the table. Soon after, no-one else you are with will have energy for any conversation at all, because any slight point of interest will set the foghorn off.
The "Book of Quotes". You know this guy. This guy watches television. And movies. Lots of them. Based on their contributions to every conversation ever, this is all they do. Talking about politics? Simpsons quote. Talking about science? Hangover quote. Talking about how much you hate people that endlessly quote movies and shows rather than coming up with their own contribution to the discourse in a vain attempt to appear intelligent and culturally aware but really they just come off as derivative and thoughtless because half the time the quote isn't even relevant or insightful? Monty Python quote.
The "A Little Too Rude". This person doesn't quite know where the line is, but that won't stop them making a dead baby joke at your Nan's birthday. Or telling everyone about that time they got really drunk and forgot to wear a condom, but it was OK because he pulled out in time in a crowded restaurant. This person makes you want to curl up into the foetal position until the awkwardness goes away forever.
The "Annoying Voice". This person has an annoying voice. Nothing they say is funny or interesting because they have an annoying voice.
The "Poor Timing". This person may be funny if they weren't just that little bit off the mark. Maybe the conversation had just moved on and it's an awkward throwback. Maybe they said something predicting where the conversation was going before it got there. Whatever the reason, this terminally rhythm-free dolt will disrupt the internal flow of any conversation they're a part of, making everyone they're around have to try a little harder just to keep the conversation running.
Think about anyone you know that is terminally unfunny. They fit into one (or more) of these categories, without fail. Or they will now, at least, because you won't be able to stop thinking about what kind of unfunny they are. My work here is done.