"This potato masher sucks."
This statement is guaranteed to send my siblings and me into gales of laughter, but other people just don't get it. That's the problem is much of our humor: it's very situational.
Many things are funny because we can relate them to something that had happened previously in our lives. The trick to writing humor is to make sure that what you're writing is funny to someone besides yourself and/or others who’ve had a similar life experience. You want it to be funny to everyone--or at least to the majority of your readers.
I remember a writer friend lamenting on the fact that she wrote a scene she thought was hilarious, her sisters felt the same way, but nobody else thought it was funny. Finally, the writer realized that it was funny to her sisters and her because it related to an event that happened to them in their childhood. Nobody else thought it was funny because they hadn't lived through that experience. She had a sucky potato masher.
You Just Had To Be There
I could tell you the story about what makes the potato masher statement so funny, but it'll get lost in the translation. It was hilarious because of the time, place and person who said it, but if you weren't there, you're not going to get it. Frankly, when I tell someone the story, it doesn't even sound all that funny to me anymore, but I still giggle when someone in my family makes the infamous potato masher pronouncement. But I realize that if a character in one of my books would say it, no one but my family would think it was funny.
There's a scene I wrote where the heroine asks for directions. After a very detailed description of how to get there involving turning left where Anderson's barn used to be and right where that oak tree got struck by lightning ten years ago, she left just as confused as she arrived. This is funny because just about everyone has experienced getting bad directions at some time in their lives or, possibly, they recognize themselves in the direction giver. Either way, giving and receiving bad directions is something everyone can relate to.
I Get It. It Just Wasn't Funny
The worst scenario in writing humor is when what you write just isn't funny. Repeating the joke to force your audience to laugh isn't going to work either. If it wasn't funny the first time, it's not going to get any better the second--or third. Of course, the opposite of this can be true as well. One of the things I enjoy about reading the latest of Janet Evanovich's Stephanie Plum books is finding out which new way and hilarious way Stephanie's car is going to be blown up this time. It became an inside joke between the author and her readers--and who doesn't love to be on the inside of an inside joke?
Everyone doesn't find the same things funny, which is why there are so many different kinds of comedy out there. People become known for their comedic style--there's a difference between reading a Jennifer Crusie book and one by Katie MacAlister. They're both hilarious, but in different ways. The trick to writing comedy is to find out what you do best, make sure your reader can relate and have a good time.