Laughter A paper on laughter by Robert Provine, published in American Scientist. Certainly, there are inherent differences in people's tendency to laugh, but, as you suggest, there are also individual differences based on sociality. Laughter is a social signal, and those spending more time in social settings are more likely to laugh. Numerous times when my wife starts to cry I start to laugh. This, until recently, has been isolated to my wife but last week has also happened with my 10 year old daughter.
Many people laugh until they cry, or, at least, tear.
In the case you describe, your brain is laughing in response to your wife's crying, even though it's not your intention. Inappropriate crying is also common, but has fewer associated social sanctions. I know that they don't mean to laugh, but it just happens. They are helping the person up but yet are laughing at the same time. This puzzles me.