I usually notice, when someone falls, slips or met some kind of misfortune, his own friends (and others) laugh at hapless person ( though they may be helping him to recover at the same time). Does not that tell that humans are generally sadistic in nature?
As Plato said: That we humans enjoy observing others' misery (Thanks Nitish).
But I am sure there is some other reason in such kind of humour, other than human sadism.
Humour is invoked when a seemingly natural course of action was going on, a sudden unexpected occurrence intercedes, and if the outcome is not fatal, then we feel relieved, and hence humour arises, because as we build our worries over the seemingly dangerous incident, but the outcome did not necessitate the worries, we have to release that tension of worries into something, in another form, so it gets transformed into concern and humour forms. Whether concern is higher or humour gets the better part, that would depend on many other factors like the mood, nature of incident, time of occurrence, etc.
So, don't feel guilty when you laugh at your friend's misfortune. After all, you are only human.