This troubles me, are you generally a happy person and just don't laugh because nothing is genuinely funny or do you struggle finding things humorous because you're a glass half empty kind of person?
You know when they say "laughter is the best medicine", well they're not lying. Laughing releases endorphins as well as dopamine's which make you feel good, as well as some other chemicals that aid stress-relief, tissue regeneration and formation of neuro-receptors in your brain.
Laughing gives you super healing and makes you smart.
Not to mention prolongs longevity.
For anybody with a negative philosophical standpoint: (Glass half empty)
When you're alone, just laugh, fake laugh pretend you've just seen the funniest thing.
Then think about how silly you must sound and look, and laugh at yourself laughing at yourself. It's honestly hilarious and your mood will shift, you'll become much more positive in life the more often you laugh at nothing.