What makes you lose your sense of humour?

Being around negative people.

Depression.

Callous disrespect of others, or worse, vulgarity in the presence of small children. 

Very little.

I have to say ,it's no joke answering question like this.....It's no Joker acting the Fool at all actually...:) D 

Racism and racist jokes. Jokes about 9/11, the holocaust, other significant tragedies, and mass genocides. When people say things like, “that’s retarded.”

*Edit* After reading another answer i must include rape, molestation, and any other form of abuse.  This post was edited by Rizz at September 7, 2018 1:41 PM MDT

im pretty much ok with any type of joke except for rape/molestation or any other type of sexual crime 

most people know better than to joke about that kind of stuff but you always have those edgelords that think it’s funny 

when bad stuff happens

Me personally?
Not understanding the joke - could be due to cultural difference or lack of background knowledge.
When I suspect that the intent of the joke is to put someone or some group of people down.
(Politicians excepted because I believe lampooning those in power is one way to hold them accountable, encourage thinking, and is healthy for a democracy.)

For people in general?
Being in a near constant state of anger or depression.

Somebody said depression. I pick that.