If a person is serious about somethin' and you take it in a light way or make a joke out of it, he/she would definitely feel ridiculed and mocked at. Hence, one should choose the right moment to tell a joke. We cannot simply tell jokes all the time 'cuz there are times when people needs serious help or answers to their problems and theleast thing they would like is a person mockin' them irresponsibly.. . Another reason is that sometimes the joke embarrasses a person and as you may figure it out, no one likes to be embarrassed before his/her friends. The joke sometimes is not a light one but harsh and painful and so it becomes hard for a person to take it easily and with a sportin' spirit :)