When someone requires another person to do something, they sometimes use "must" in order to emphasize the fact that they have to comply. Must is more forceful than need, when you are giving directions or commands to someone else. For example, "You must remove your shoes after entering the building". "Need" may be used when making suggestions that you think the other person should follow. Since it's a suggestion, not a command, the word "need" gives the person more room to choose other options. For example, you may tell a friend that you're upset by something someone said to you. While giving advice, your friend might say, "You need to tell him/her that you're upset".