Always begin with a capital letter. I agree with fdmaxey with additional explanation. I hope it helps. 1) Why are you so selfish? - this has something to do with the individual's personality. It describes his/her character in general. Like, "why are you so kind, thoughtful, generous, etc. 2) Why are you being so selfish? - this is about the person's reaction at the moment. It's not permanent. It's how he/she responds at a certain situation.

The first sentence says that selfishness is a part of that person's being - that they are generally selfish all the time. The second sentence is not as harsh. It implies that while the person in question may not be selfish all the time, his or her actions in this particular instance are selfish.

in the second sentence when you say "being" you are usually implying that they are acting that way at the current moment. if you ask for something, and they say no, then you say "your BEING selfish" or at this current time. as for if you dont put BEING, then you are implying that they are always that way. if you always deal with a selfish person you might say "Man you're so selfish" "Man you're always selfish" "Man you should lighten up, and stop being selfish all the time" if you want to put it easier, being is SLIGHTLY less rude, as you are implying now and not all the time. so easy, if its a one time deal, just say BEING. if you know them and its all the time, then dont use it.

i don't have facts but my impression on the sentences is that #1 is that you consider his selfishness a fact or you believe he is. while #2 focuses on his action and implies that he is not selfish before.