Why are people tell lie in life?

It is a pity!Sometimes people have to tell a lie. But the lie is a white lie,not a black lie.For exemple, I have a friend,she loves her grandmother very much.Unfortunately,her grandmother was killed at a traffic accident.At this time, she was preparing an important exam far away from her home.To consider her feelings, we decided not to tell her the truth when she entrusted us to take good care of her grandmother.I do know it is wrong,but I have to.If you were I,what will you do?I'm sure you may do the same.