You need to be silent sometimes because your gf will remember the time you missed her call lol
For those who say that this is improper or wrong: not really. It is not a full sentence, true, but thats because these are both exclamatory sentences (like "Who?" or "Stop!"). No, it does not give a reason, but in my experience this is rather a common sentence to reinforce an order. Think of it as a shorter form of "Why do you need to do this? Because I told you to/because I said so!"