It is not the same to LAUGH WITH SOMEONE, than to LAUGH AT SOMEONE.

Laughter is a natural response generally produced by the unexpected, as @Undisciplined states, or by the absurd, although this is not necessarily bad or inappropriate. Dopamine, as a chemical substance associated with neurotransmission, acts both in the area of joy and sadness, so it can be an involuntary response.

@Coinsreporter points out something important, in principle the person can respond by laughing when we fall, but they quickly help when they notice that we have been hurt. But let's think that people whose job is to treat medical emergencies do not have the same reaction, if someone falls, the instinctive thing is to help and if after all something funny comes out then they laugh at it, then the condition of mental training and alertness influence our reaction.

Of course that has nothing to do with other things, such as ENJOYING the suffering of others, sadism, child abuse or things like that. The saddest thing is that this has more views than educational materials on any subject. This shows human degradation.