common sense is actually not common

why are there ppl who have no common sense?!
eg be late for 30 mins, hving the whole group of 15-20 ppl waiting for her? and hv no good reason for being late
eg why some ppl dont clean up the mess after they use the kitchen/tea room?

all these ppl are not small kids! they are all uni students or those who are already working and are at least graduated from uni.