Perhaps such a transformation is rare, but just as the funniest man you knew has recently become a boring dud, the fact is that anyone can learn to be humorous.
Wit and Humor
If you're negative and pessimistic all the time, it's going to be hard to find anything humorous and furthermore hard to be funny. Learn to appreciate things that happen around you. If good things happen, you'll be happy. If bad things happen, at least you get an amusing story to tell! It's how you see a situation that matters.
Often it's the delivery, that gives your girlfriend a laugh. Ever read the transcripts of episodes of the famous sitcom Friends? Chandler doesn't seem all that funny there, does he? That's because what he may lack in substance, he compensates for with his delivery. Hence, if you want to make people laugh, learn how to deliver the line so that it sounds best.
The element of surprise can often play a key role in comedy. In-your-face humor can often get boring and repetitive. Hence, it is always good to have the surprise element. It gives a larger impact to a joke or an anecdote you just told.
An Ace Up Your Sleeve
What if you don't have anything entertaining to say? If such a situation arises, it is better to be armed with a good joke or an anecdote just in case. Keep a hilarious story up your sleeve, so in case you're not feeling at your funniest at the time, you still don't lose out on the humor quotient.
You need to be prepared for anything since some things are only funny in certain situations. Timing is the key in comedy. Always be ready to make use of the current situation.
Observation is extremely important in the quest to be a funny man. Because if you're not naturally humorous, then you might have to copy the style of the jokes that someone else comes up with. So observe what a comedian is saying, how he's saying it, and what the response is like.